leds: leds-pm8xxx: Add CABC support for backlight control

Content adaptive backlight control (CABC) support is needed to
control backlight by external PWM.  It can be enabled by setting
"cabc_en = true" in the platform data wled_cfg structure.

CRs-fixed: 379347
Change-Id: If30defd3a3ca61283233c97ab26f255a3a62ad7c
Signed-off-by: Amy Maloche <amaloche@codeaurora.org>