dts/riscv/microchip: add missing cpu nodes compats in `mpfs.dtsi`

The cores used in the `mpfs.dtsi` file are:
* 1x SiFive E51 (RV32)
* 4x SiFive U54 (RV64)

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>