msm: qpnp-power-on: add debugfs support

Add debugfs support for the user to write data to a file for
qpnp-power-on instances. If the power on reason is SMPL or the power off
reason is UVLO, panic the device immediately after the user write data
to uvlo_panic file in debugfs. Otherwise, do nothing. Also, the user could
read back the uvlo_panic value through debugfs to verify previous setting.

Change-Id: I8e49ac1e3096162d76812019f428d87e2f073098
Signed-off-by: Ke Liu <keliu@codeaurora.org>