input: qpnp-power-on: remove unconditional S3 reset configuration

Some PMIC PON peripherals are read-only for the application
processor.  When the qpnp-power-on driver attempts to write to
the PON S3 reset control registers, the writes fail which causes
PON device probing to fail.

Fix this by only configuring S3 reset when S3 configuration
properties are specified in device tree.

Change-Id: Ic59ae8a10a4976daba920ad079eaca3f648ff200
Signed-off-by: David Collins <collinsd@codeaurora.org>