regulator: dt-bindings: add documentation for s2mpu05-pmic regulators

S2MPU05 is a PMIC found in Exynos7870 devices, which controls voltage
regulators (21 LDOs and 5 BUCKs). Provide documentation for devicetree
definitions, regulator naming patterns, etc.

Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250301-exynos7870-pmic-regulators-v3-1-808d0b47a564@disroot.org


Signed-off-by: Lee Jones <lee@kernel.org>