ARM: dts: tbs2910: rename MMC node aliases

to be consistent with kernel versions up to v5.9 (mmc aliases not used here).
usdhc1 is not wired up on this board and therefore cannot be used.
Start mmc aliases with usdhc2.

Signed-off-by: Soeren Moch <smoch@web.de>
Cc: stable@vger.kernel.org                # 5.10.x
Signed-off-by: Shawn Guo <shawnguo@kernel.org>