[ARM] 4907/1: [AT91] SAM9/CAP9 reset reason

The Reset controller on the SAM9/CAP9 processors will store the reason
for the last system reset.
On startup, display this information (wakeup signal, RTT alarm,
watchdog reset, user reset, etc)

Based on patch from David Brownell.

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>