dt-bindings: arm: cpus: Document pu-supply

The i.MX6Q Reference Manual describes the three digital LDO regulators
as follows:

"10.4.1.1.1
Digital LDO Regulators
The integrated PMU includes three digital LDO regulators: LDO_ARM, LDO_PU, and
LDO_SOC. These regulators provide power to the ARM_Core power domain, the
combined VPU, IPU and GPU power domain, and the rest of the SoC logic (except
always-ON SNVS domain)."

imx6dl.dtsi uses the correct names to describe these supplies:

arm-supply = <&reg_arm>;
pu-supply = <&reg_pu>;
soc-supply = <&reg_soc>;

'arm-supply' and 'soc-supply' are already documented, but 'pu-supply' is not.

Document the 'pu-supply' property and set it to deprecated.

This fixes the following dt-schema warning:

cpu@1 (arm,cortex-a9): Unevaluated properties are not allowed ('pu-supply' was unexpected)

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250907152513.590218-1-festevam@gmail.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>