ARM: at91: switch to generic board for at91sam9260 series boards

Includes at91sam9260ek, at91sam9g20ek and at91sam9xeek boards.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
ARM: at91: mmc portA support is only for at91sam9g20ek_2mmc board

The MMC support will enable MCI port A. Which is only exist for 2mmc
board. So move this for 2mmc board.

Since MMC has confliction with Dataflash in the CLK pin, so this patch
will make dataflash works for at91sam9260ek board.

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