rtc: at91sam9 RTC support (RTT and/or RTC)

rtc: at91sam9 RTC support (RTT and/or RTC)

AT91sam9 RTC support, primarily in the form of an RTT-as-RTC driver that was
extracted from 2.6.23-at91 patch and updated:

 - Relies on now-merged platform updates, which associate the RTT
   hardware address with each RTT and use the "at91_rtt" name.

 - RTC framework related fixes and cleanups, notably:
    * removed now-needless suspend/resume clock offset logic
    * alarm read/write now respects the "enabled" flag
    * suspend always disables update irqs
    * shutdown (and startup) disables all irqs

 - Misc cleanup:
    * use dev_*() messaging
    * add comments
    * remove globals,
    * ... etc

 - Don't force use of RTT0 and GPBR0.  Either resource may need
   to be used for other purposes (like NO_HZ support).

 - Update "AT91RM9200 RTC" Kconfig to allow it on SAM9RL chips
   (it has both RTT and RTC).

Driver binding uses bus_find_device() to avoid needing any kind of "timer
library" code when there's more than one RTT module.  (This timer can be used
as an RTC, to support NO_HZ operation, or potentially for other stuff.  The
choice is a per-system policy.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Michel Benoit <murpme@gmail.com>
Cc: Nicolas Ferre <nicolas.ferre@rfo.atmel.com>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>