ARM: AM43x: Fix MAX_RAM_BANK_SIZE

On AM437x-GP Evm there is 2GB of DDR3 memory available as stated in
AM437x GP EVM HardwareUser's guide http://www.ti.com/lit/ug/spruhw7/spruhw7.pdf

.
But MAX_RAM_BANK_SIZE is defined as 1GB.
Fixing MAX_RAM_BANK_SIZE to 2GB on AM43xx.

Reported-by: Shivasharan Nagalikar <shivasharan.nagalikar@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>