riscv: dts: canaan: fix mmc node names

The newly-converted-to-dt-schema binding expects the mmc node name to be
'^mmc(@.*)?$' so align the devicetree with the schema.

Tested-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220705215213.1802496-8-mail@conchuod.ie


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>