ARM: dts: msm-pm8019: Switch VADC to polling

Switch VADC to use polling mode to check if the
conversion request is complete by checking the status
bits for the corresponding VADC registers to
indicate the conversion is complete.

Change-Id: I7a28d0e36bbbad465cbbfe716069d88af88534ba
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>