dt-bindings: mfd: x-powers,axp152: Document the AXP15060 variant

The X-Powers AXP15060 is a PMIC seen on Starfive Visionfive 2 board. Add
relative compatible item and CPUSLDO support and disables DC-DC
frequency setting for it.

Signed-off-by: Shengyu Qu <wiagn233@outlook.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/TY3P286MB261177CF7AA2959BD9517DA998609@TY3P286MB2611.JPNP286.PROD.OUTLOOK.COM