mailbox: mpfs: drop POLARFIRE from ARCH_MICROCHIP_POLARFIRE

mailbox: mpfs: drop POLARFIRE from ARCH_MICROCHIP_POLARFIRE

The ARCH_MICROCHIP symbol has been defined for some time on RISCV, as a
replacement for ARCH_MICROCHIP_POLARFIRE since there are now other
Microchip RISC-V products. Drop the POLARFIRE from
ARCH_MICROCHIP_POLARFIRE in the POLARFIRE_SOC_MAILBOX Kconfig entry
since the newly added pic64gx also uses the mailbox and it is one of the
few users of ARCH_MICROCHIP_POLARFIRE left in the tree.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>