mfd: Add support for sun6i PRCM (Power/Reset/Clock Management) unit

The PRCM (Power/Reset/Clock Management) block exposes several subdevices
in different subsystems (clk, reset ...)

Add basic support for the PRCM unit with clk (AR100, AHB0, and APB0 clks)
and reset controller subdevices.

Other subdevices might be added later (if needed).

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>