dt-bindings: mfd: qcom-i2c-pmic: Add bindings

Add snapshot of the dt-bindings of the qcom-i2c-pmic driver as of
msm-4.14 'commit 7e16ff6cdde5 ("defconfig: qcs405: enable WLAN configs
for 32-bit QCS405")'.

Change-Id: I82bce50edebd75bf016feca76b9c7db3bc03a6d5
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>