atmel_serial oops when peripheral clock misconfigured

In lk 3.7.0-rc4 when a peripheral clock is not found for a serial port the atmel_serial driver brings down the kernel with an oops during boot-up. This impacts the Atmel AT91 family of MCUs.

For example, arch/arm/mach-at91/at91sam9x5.c does not specify properly the peripheral clocks for the UTXD0/URXD0 and UTXD1/URXD1 serial ports. Selecting either of those ports in a dts file will crash the kernel. at91sam9x5.c needs to be fixed but stopping atmel_serial crashing the kernel is more urgent.

Patch attached for your consideration.

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