soc: apple: Add driver for Apple PMGR power state controls

Implements genpd and reset providers for downstream devices. Each
instance of the driver binds to a single register and represents a
single SoC power domain.

The driver does not currently implement all features (clockgate-only
state, misc flags), but we declare the respective registers for
documentation purposes. These features will be added as they become
useful for downstream devices.

This also creates the apple/soc tree and Kconfig submenu.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hector Martin <marcan@marcan.st>