riscv: hw: Remove not needed PLIC properties in device tree

riscv: hw: Remove not needed PLIC properties in device tree

This removes "reg-names" and "riscv,max-priority" properties of the
PLIC node from device tree.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jonathan Behrens <fintelia@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>