leds: leds-qpnp: Add flash_wa regulator support in flash led driver

Add flash_wa regulator support in flash led driver. The regulator raise
the SMBB over temperature threshold before flash enabled, and restore to
default setting after flash disable.

CRs-Fixed: 697457
Change-Id: I3f376be749704b2c34c3ee99b4c155d28ad2b6b5
Signed-off-by: Chunmei Cai <ccai@codeaurora.org>