ARM: omap: Allow to set mmc devname used for booting
This adds a omap_set_bootmmc_devname function which can be used
to specify the MMC boot device. In an xload configuration this
is then used to load the next stage. In a regular build this is
used to mount /boot.
Also a device_detect_by_name is added to allow to disable probing
for sd/mmc cards during startup.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>