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

Add bindings for the I2C PMIC Controller.  This device handles
interrupts from an I2C PMIC and supports various subnodes which
each correspond to a PMIC peripheral (e.g. LDO regulator, GPIO,
etc).

Change-Id: I82bce50edebd75bf016feca76b9c7db3bc03a6d5