ARM: OMAP: Mark Beagleboard-xM MMC bus as 4-bit

On Beagleboard-xM (all revisions) only MMC1_DAT0-MMC1_DAT3 are wired up.
Tested on Beagleboard-xM Rev C1 and Beagleboard Rev B4.

Signed-off-by: Russ Dill <Russ.Dill@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>