power: pmic: Add driver for ST-Ericsson AB8500 via PRCMU

All devices based on ST-Ericsson Ux500 use a PMIC similar to AB8500
(Analog Baseband). There is AB8500, AB8505, AB9540 and AB8540
although in practice only AB8500 and AB8505 are relevant since the
platforms with AB9540 and AB8540 were cancelled and never used in
production.

In general, the AB8500 PMIC uses I2C as control interface, where the
different register banks are represented as separate I2C devices.
However, in practice AB8500 is always connected to a special I2C bus
on the DB8500 SoC that is controlled by the power/reset/clock
management unit (PRCMU) firmware.

Add a simple driver that allows reading/writing registers of the
AB8500 PMIC. The driver directly accesses registers from the PRCMU
parent device (represented by syscon in U-Boot). Abstracting it
further (e.g. with the i2c uclass) would not provide any advantage
because the PRCMU I2C bus is always just connected to AB8500 and
vice-versa.

The ab8500.h header is mostly taken as-is from Linux (with some
minor adjustments) to allow using similar code in both Linux and
U-Boot.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>