ARM: at91: merge SRAM Memory banks thanks to mirroring

On at91sam9260 and at91sam9g20 the SRAM banks are mirrored. We can
merge them together to be able to have bigger and continuous
internal RAM.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>