leds: leds-qpnp-flash: create flash LED driver

Add driver for flash LED module which is part of PMIC.
Flash LED supports flash light and torch light for camera
and other userspace apps.

Change-Id: Ie1e2df2995e6acdffa3d183ec924599d9f4b3b6e
Signed-off-by: Chun Zhang <chunz@codeaurora.org>