input: synaptics_i2c_rmi4: Add low power mode

Enable regulators to go into low power mode when
device enters suspend, to save power.  In resume,
return regulators to full power.

Change-Id: I6f5a6d398732b2e23aeba81b3569ae2c081f31e5
Signed-off-by: Amy Maloche <amaloche@codeaurora.org>