ARM: dts: am33xx: add aliases for mmc interfaces

Without DT aliases, the numbering of mmc interfaces is unpredictable.
Adding them makes it possible to refer to devices consistently.  The
popular suggestion to use UUIDs obviously doesn't work with a blank
device fresh from the factory.

See commit fa2d0aa9

 ("mmc: core: Allow setting slot index via
device tree alias") for more discussion.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>