dt-bindings: iio: Add PMX65 PMIC7 ADC support

PMX65 belongs to the PMIC7 family of chips that makes use of PMK8350 to
read ADC data from their onboard channels.

Change-Id: Id0c51bc6c69568d6ea67e5067c8c81ebf6719b63
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>