pmic: pmic_hi6553: Add a driver for the hi6553 pmic found on hikey board.

This adds a simple pmic driver for the hi6553 pmic which is used in
conjunction with the hi6220 SoC on the hikey board. Eventually this
driver will be updated to be a proper UCLASS PMIC driver which
can parse the voltages direct from device tree.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>