at91sam9263ek: add mmc support

Add support for using the Atmel MCI driver on at91sam9263ek.
This change is modeled after the existing at91sam9260ek support.

Please note that this hooks up slot1 (MCI1) for SD. Not both.

Tested with at91bootstrap and u-boot on dataflash in slot 0
and fat-formatted 8GB SDHC in slot 1 on first revision
at91sam9263ek (which must use dataflash in slot0 to boot).

CONFIG_ATMEL_MCI_PORTB not tested.

Signed-off-by: Andreas Henriksson <andreas.henriksson@endian.se>
[remove empty line]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>