dt-bindings: mfd: Add bindings for the keys as subnode of PMIC

This patch adds documentation for device tree bindings for keys support
as the subnode of MT6397/MT6323 PMIC.

Signed-off-by: Chen Zhong <chen.zhong@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>