leds: qpnp-flash-v2: Add support for LPG strobe

Flash LED3 can be configured for LPG strobe in addition to the
existing HW or SW strobe types. LPG strobe works the same way
as HW strobe except the strobing is controlled by LPG output
instead of a GPIO. To accommodate this, device tree property
"qcom,hw-strobe-sel" got renamed to "qcom,strobe-sel".

While at it, configure hardware strobe option only when the
option is specified through device tree.

Change-Id: I9351d7b7cd8c57ff7707a4ada6c5bdfa42772390
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>