RSTC_KEY_UNLOCK: use defined magic value

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
at91sam9x5ek: AT91C_DDRC2_EBISHARE bit cleared in DDR configuration

Comment about the use of AT91C_DDRC2_EBISHARE bit in DDR controller
configuration and the reason why it is cleared when considering the
AT91SAM9x5-EK board case.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>