dt-bindings: clock: sun55i-a523-ccu: Add A523 MCU CCU clock controller

dt-bindings: clock: sun55i-a523-ccu: Add A523 MCU CCU clock controller

There are four clock controllers in the A523 SoC. The existing binding
already covers two of them that are critical for basic operation. The
remaining ones are the MCU clock controller and CPU PLL clock
controller.

Add a description for the MCU CCU. This unit controls and provides
clocks to the MCU (RISC-V) subsystem and peripherals meant to operate
under low power conditions.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250911174710.3149589-3-wens@kernel.org
Signed-off-by: Chen-Yu Tsai <wens@csie.org>