MMC: AU1xMMC: Allow platforms to disable host capabilities

Although the hardware supports a 4/8bit SD interface and the driver
unconditionally advertises all hardware caps to the MMC core, not all
datalines may actually be wired up.  This patch introduces another
field to au1xmmc platform data allowing platforms to disable certain
advanced host controller features.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
To: linux-mmc@vger.kernel.org
CC: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/460/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>