mailbox: mpfs: convert SOC_MICROCHIP_POLARFIRE to ARCH_MICROCHIP_POLARFIRE

As part of converting RISC-V SOC_FOO symbols to ARCH_FOO to match the
use of such symbols on other architectures, convert the Microchip FPGA
mailbox driver to use the new symbol.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>