dt-bindings: mfd: syscon: Document the control-scb syscon on PolarFire SoC

dt-bindings: mfd: syscon: Document the control-scb syscon on PolarFire SoC

The "control-scb" region, contains the "tvs" temperature and voltage
sensors and the control/status registers for the system controller's
mailbox. The mailbox has a dedicated node, so there's no need for a
child node describing it, looking the syscon up by compatible is
sufficient.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250901-shorten-yahoo-223aeaecd290@spud
Signed-off-by: Lee Jones <lee@kernel.org>