dt-bindings: soc: microchip: document the simple-mfd syscon on PolarFire SoC

dt-bindings: soc: microchip: document the simple-mfd syscon on PolarFire SoC

"mss-top-sysreg" contains clocks, pinctrl, resets, an interrupt controller
and more. At this point, only the reset controller child is described as
that's all that is described by the existing bindings.
The clock controller already has a dedicated node, and will retain it as
there are other clock regions, so like the mailbox, a compatible-based
lookup of the syscon is sufficient to keep the clock driver working as
before, so no child is needed. There's also an interrupt multiplexing
service provided by this syscon, for which there is work in progress at
[1].

Link: https://lore.kernel.org/linux-gpio/20240723-uncouple-enforcer-7c48e4a4fefe@wendy/
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>