leds: Add support to limit max_brightness of LCD

Make /sys/class/leds/lcd-brightness/max_brightness writable. This
will help limiting the max brightness and control the heat generated
by the LCD.

Change-Id: Ieb297f72f5fc26c19ca09c542e3a4140d01865d4
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>