leds: leds-qpnp-flash: add current derate control and expose max current

Add sysfs entries for enabling/disabling current derate control,
and exposing the max allowed PMIC current for "flash LEDs" to userspace.

Change-Id: Ia9c6269923487f9cf356e0bd68f32f89a4b820e6
Signed-off-by: Jigarkumar Kishorkumar Zala <j_zala@codeaurora.org>