Add MMC aliases to reference hardware devicetree

To ensure MMC device naming is deterministic after the change to
asynchronous probing in upstream commit 7320915c8861, add the
required MMC device aliases.  This matches what was done for the
Salvator-X boards in d765a4f3 in upstream mainline (or e6d95f35
in linux-5.10.y).

Bug-AGL: SPEC-4432

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I7d86c27a9547daa6017d72e01820d739ef0d07d9