mailbox: change mailbox-mpfs compatible string

mailbox: change mailbox-mpfs compatible string

[ Upstream commit f10b1fc0161cd99e54c5687fcc63368aa255e05e ]

The Polarfire SoC is currently using two different compatible string
prefixes. Fix this by changing "polarfire-soc-*" strings to "mpfs-*" in
its system controller in order to match the compatible string used in
the soc binding and device tree.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>