ARM: dts: am33xx: add aliases for mmc interfaces

[ Upstream commit 9bbce32a20d6a72c767a7f85fd6127babd1410ac ]

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 fa2d0aa96941 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>