ARM: dts: msm: Add measurement interval mode for PMI8994 VADC

Measurement interval mode supports recurring measurement on
any one VADC channel. Both Single and recurring measurement
will be supported on the VADC_USR for PMI8994.

Change-Id: I83e0e0788dfe7663bd050c6115adae978225083f
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>