[ARM] at91: correct value for AT91_RSTC_KEY

- Remove a duplicated define for AT91_RSTC_KEY
- Set AT91_RSTC_KEY to the correct value
- Replace the hardcoded keys in at91sam9620.c and at91sam9261.c
  by AT91_RSTC_KEY

Signed-off-by: Jan Altenberg <jan@linutronix.de>
Acked-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>