riscv: dts: microchip: refactor icicle kit device tree

Assorted minor changes to the MPFS/Icicle kit device tree:

- rename serial to mmuart to match microchip documentation
- move phy0 inside mac1 node to match phy configuration
- add labels where missing (cpus, cache controller)
- add missing address cells & interrupts to MACs

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>