leds: pm8xxx: add pwm control sysfs files

PWM(frequency) is controlled by userspace

Change-Id: I6a3194de0b843841cdf56941dbcc43fcdc3a4f42
leds: pm8xxx: adjust the max brightness

This maps android brightness level 0 to 255 into driver backlight
level 0 to board max brightness with roudning

Change-Id: I760881fc25d7799afe55df8d213533c19432e81f
leds: pm8xxx: change adjust brightness routine

add pwm_adjust_brightness field for current control.

Change-Id: If0ba79324fcf50334f88f006a8ff30b59b7d4982
leds: pm8xxx: add calculation of blinking period and on/off

Change-Id: I718f506051ef367db08c7c8413e536cd5da22a95
leds: pm8xxx: change calculation of blinking period and on/off timing

Change-Id: I6adae65ae692c0a3d79bc45356ba8f880b3d00f3
Signed-off-by: Iliyan Malchev <malchev@google.com>