arm64: dts: exynos: gs101-pixel-common: add main PMIC node

arm64: dts: exynos: gs101-pixel-common: add main PMIC node

On Pixel 6 (and Pro), a Samsung S2MPG10 is used as main PMIC, which
contains the following functional blocks:
    * common / speedy interface
    * regulators
    * 3 clock outputs
    * RTC
    * power meters
    * GPIO interfaces

This change enables the PMIC itself and the RTC. We're still working on
the remaining parts or waiting for bindings to be merged, hence only a
small subset of the functional is being enabled.

The regulators fall into the same category (still being finalised), but
since the binding requires a 'regulators' node, an empty node is being
added to avoid validation errors at this stage.

Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250627-gs101-reboot3-v1-2-c3ae49657b1f@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>