arm64: dts: ti: ti-k3*: Introduce aliases for mmc nodes

Since probe order of mmc can vary depending on device tree dependencies,
Lets try and introduce a consistent definition of what mmc0, 1 are
across platforms.

NOTE: Certain platforms may choose to have overrides due to various
legacy reasons, we permit that in the board specific alias definition.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Aswath Govindraju <a-govindraju@ti.com>
Link: https://lore.kernel.org/r/20210915135415.5706-1-nm@ti.com