dt-bindings: mailbox: fix the mpfs' reg property

The "data" region of the PolarFire SoC's system controller mailbox is
not one continuous register space - the system controller's QSPI sits
between the control and data registers. Split the "data" reg into two
parts: "data" & "control".

Fixes: 21355623 ("dt-bindings: soc/microchip: update syscontroller compatibles")
Fixes: ed9543d6 ("dt-bindings: add bindings for polarfire soc mailbox")
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>