omap5_common: Re-work mmc boot to try SD and eMMC, correct root device

OMAP5 boards may have both eMMC (on MMC2) and an SD slot (on MMC1).  We
Update the default bootcmd to match what happens on AM335x where we try
SD first, and then eMMC.  In this case however, the hardware layout used
for powering both of these means that in the kernel eMMC shall be found
first as it is powered by a fixed regulator and SD found second as SD is
powered via the palmas which will result in deferred probing.

Tested-by: Aparna Balasubramanian <aparnab@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>