v2 rtc-at91rm9200: add support for at91sam9x5

An unspecified number of SoCs from the Atmel's at91sam9x5 family including the at91sam9g25 have a broken AT91_RTC_IMR (interrupt mask) register which always returns zero.

If this bug can be neutralized, then the existing rtc-at91rm9200 driver will work. On the other hand, leaving this bug in place, and starting the RTC causes unhandled interrupts which result in the "SYS" interrupt being disabled. And that takes down several other interrupts wired or-ed through the SYS interrupt including the DBG port.

This is the second version of this patch, and is against lk 3.10.0-rc2 .

ChangeLog:
- checks in probe() function if AT91_RTC_IMR is broken. If so, uses a shadow imr
- apart from that check, SoCs with a good IMR register take the same paths through rtc-at91rm9200.c as before
- SoCs with a broken IMR take a spinlock while changing the state of IER or IDR (and the shadow), mainly to disable interrupts **.

** similar technique used in arch/arm/mach-at91/clock.c

Tested on an Aria G25 (at91sam9g25 based).

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Your patch is almost identical to the one I submitted about two months ago in this thread:

	https://lkml.org/lkml/2013/4/3/166

except that you do not use DT to the detect SoCs with broken IMR. (Your patch is also lacking the register-read-back heuristic to make sure the interrupts have been disabled before you update the mask.)

Are there sam9x5 SoCs with non-broken RTC_IMR which would prevent us from using DT for detection? If so, I'd rather see your test for broken-IMR implemented on top of my implementation.

Thanks,
Johan