drivers: PL011: add ARM SBSA Generic UART support

This is the second revision of the SBSA UART support series. It is now based on 4.0-rc2 and Dave's PL011 fixes[2], which fixes all problems seen on the fast models and on some hardware. I also added support for reporting back the actual baudrate to userland, which needs to be passed in via the device tree by the firmware now. An attempt to change that value will be ignored by the driver, sane userland software (like stty) also rightfully complains about not being able to change it:

# stty < /dev/ttyAMA1 | head -n 1
speed 115200 baud; line = 0;
# stty 38400 < /dev/ttyAMA1
stty: standard input: cannot perform all requested operations
# stty < /dev/ttyAMA1 | head -n 1
speed 115200 baud; line = 0;

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The ARM Server Base System Architecture[1] document describes a generic UART which is a subset of the PL011 UART.
It lacks DMA support, baud rate control and modem status line control, among other things.
The idea is to move the UART initialization and setup into the firmware (which does this job today already) and let the kernel just use the UART for sending and receiving characters.

This patchset integrates support for this UART subset into the existing PL011 driver - basically by refactoring some functions and providing a new uart_ops structure for it. It also has a separate probe function to be not dependent on AMBA/PrimeCell. It provides a device tree binding, but can easily be adapted to other device configuration systems.
Beside the obvious effect of code sharing reusing most of the PL011 code has the advantage of not introducing another serial device prefix, so it can go with ttyAMA, which seems to be pretty common.

This series relies on Dave's recent PL011 fix[2], which gets rid of the loopback trick to get the UART going. There is a repo at [3] (branch sbsa-uart/v2), which has this patch already integrated.

Patch 1/10 contains a bug fix which applies to the PL011 part also, it should be considered regardless of the rest of the series. Patch 2-7 refactor some PL011 functions by splitting them up into smaller pieces, so that most of the code can be reused later by the SBSA part.
Patch 8 and 9 introduce two new properties for the vendor structure, this is for SBSA functionality which cannot be controlled by separate uart_ops members only.
Patch 10 then finally drops in the SBSA specific code, by providing a new uart_ops, vendor struct and probe function for it. Also the new device tree binding is documented.

For testing you should be able to take any hardware which has a PL011 and change the DT to use a "arm,sbsa-uart" compatible string and the baud rate with the "current-speed" property.
Of course testing with a real SBSA Generic UART is welcomed - as well as regression testing with any PL011 implementation.

Changelog v1..v2:
- rebased on top of 4.0-rc1 and Dave's newest PL011 fix [2]
- added mandatory current-speed property and report that to userland