ARM: at91: mmc portA support is only for at91sam9g20ek_2mmc board

Current the MMC support will enable MCI port A, Which is only exist
for 2mmc board.
So by default we need to disable MMC (port A) support. And only enable
it for 2mmc board. Otherwise, dataflash won't work in at91sam9260ek board
as MMC has confliction with Dataflash in the CLK pin.

Signed-off-by: Josh Wu <josh.wu@atmel.com>