msm: qpnp-power-on: Update register macros in PON driver

Update the register macros in PON driver to use the pon
variable instead of the pon->base variable to make adapting
the driver to newer hardware versions easier.

Change-Id: I4f99db77263f60e9352082337c2a73465725e5f8
Signed-off-by: Devesh Jhunjhunwala <deveshj@codeaurora.org>