mmc: arasan: Add driver for Arasan SDHCI

Add a driver for Arasan's SDHCI controller core.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Rob Herring <rob.herring@calxeda.com> [binding]
Acked-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Chris Ball <chris@printf.net>