leds: led-class: add support for max_brightness store

Add store interface for max_brightness to allow users
to change the maximum brightness to be supported by
the hardware.

Change-Id: I8b65debdc52ded24227483c4db21aaec63e27927
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>
Signed-off-by: Manaf Meethalavalappu Pallikunhi <manafm@codeaurora.org>