armv7: omap-common: Rework SPL board_mmc_init()

Since the changes in a1e56cf6 the way that we had board_mmc_init()
structured for OMAP parts (so that we always report device 0) are no
longer functional.  For now, make the case of booting from the second
device initialize both devices (we have no devices that only have the
second device as MMC).  A further rework and consolidation of the
functions should be done at a later date.

Tested on Beaglebone Black (SD and eMMC boot).

Reported-by: Vagrant Cascadian <vagrant@debian.org>
Signed-off-by: Tom Rini <trini@konsulko.com>