leds: leds-qpnp-flash: add PMIC die temp current derate feature

Add sysfs entry for enabling and disabling PMIC die temperature
derate current feature. Improve functionality for exposing
max allowed current for flash LED, with PMIC die temperature
derate current.

Change-Id: I2b17c0ca1fa0d26e5d3efd23c9a318f1c9782df0
CRs-Fixed: 793990
Signed-off-by: Jigarkumar Kishorkumar Zala <j_zala@codeaurora.org>