atmel-mci: Integrate AT91 specific definition in header file

The MCI IP is shared among AVR32 and AT91 SOCs.
AT91 has specific bit definitions in the user interface of MCI SD/MMC IP.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Pierre Ossman <pierre@ossman.eu>