soc: rockchip: power-domain: Add power domain driver

This driver is found on RK3288 SoCs.

In order to meet high performance and low power requirements, a power
management unit is designed or saving power when RK3288 in low power
mode.
The RK3288 PMU is dedicated for managing the power of the whole chip.

PMU can work in the Low Power Mode by setting bit[0] of PMU_PWRMODE_CON
register. After setting the register, PMU would enter the Low Power mode.
In the low power mode, pmu will auto power on/off the specified power
domain, send idle req to specified power domain, shut down/up pll and
so on. All of above are configurable by setting corresponding registers.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>

[replace dsb() with dsb(sy) for arm64 buildability; sy is the default,
so no functional change; adapt to per-user clocks in genpd]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>