uio_pruss: add support for devicetree and am33xx

Hi,

this series adds PRUv2 support to uio_pruss through devicetree, makes the device usable on am33xx and enables it on beaglebone black. Inspired by old patches from Matt Porter found in a downstream tree.

To archieve that this series:
* adds a flag to omap_hwmod.c to get PRUSS out of hardreset (patch 5 and 6)
* adds devicetree support to uio_pruss (patch 7 and 9)
* adds the device to the am33xx dtsi and boneblack dts (patch 12 and 13)

Bits and pieces:
* some cleanup (patch 1-4)
* take care of a fact that SRAM on am33xx is not exposed through UIO (patch 8)
* add runtime pm support to enable clocks (patch 10)
* allow the driver to be compiled on SOC_AM33XX (patch 11)

This is only tested on beaglebone black (as that's the only hardware of the PRUSS enabled families I have) with some basic GPIO and IRQ tests.

Notes:
* I just got this hardware and I don't know if this UIO PRUSS business is desired. Looking at the userspace driver I'd guess not so much ;), but this interface is there for older generations anyway, and this small series lets me use the device.
* is the hardreset thing I did there the right thing to do? I think the proper way would be a reset controller (which apparently doesn't yet exist for this SoC?) and let the driver deassert/assert on probe/remove?
* the platform device path has a clk_enable() / clk_put() calls. Are those now redundant with the introduced pm_runtime_enable() pm_runtime_disable() calls?