dt-bindings: gpu: mali-valhall: make mali-supply optional

dt-bindings: gpu: mali-valhall: make mali-supply optional

Not all platforms require the mali-supply regulator. This change removes
it from the required list in the binding schema, and make mali-supply
required for rk3588 only.

Signed-off-by: Rain Yang <jiyu.yang@nxp.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://lore.kernel.org/r/20250928090334.35389-1-jiyu.yang@oss.nxp.com