msm: qpnp-power-on: add dload_upon_uvlo module parameter

When 1 is written to this dload_on_uvlo module parameter,
BIT 7 in XVDD_RB_SPARE will be set. This XVDD-backed
register will retain its value during an UVLO event.

When the device boots up again SBL will check both
the PON reason for UVLO and if this bit is set download
mode will be entered instead of a normal boot.

One can enable this feature as follows:

  echo 1 > /sys/module/qpnp_power_on/parameters/dload_on_uvlo

Alternatively one can specify the parameter on the command line:

  qpnp_power_on.dload_on_uvlo=1

CRs-Fixed: 678229
Change-Id: I46d2d4c0586ddce9f64a02c954c37f4b367c33b2
Signed-off-by: David Keitel <dkeitel@codeaurora.org>