mfd: hi655x: Add document for hi665x PMIC

DT bindings for hisilicon HI655x PMIC chip.

Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
Signed-off-by: Fei Wang <w.f@huawei.com>
Signed-off-by: Xinwei Kong <kong.kongxinwei@hisilicon.com>
Reviewed-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>