drm/msm/dsi-staging: add new backlight type for secondary panel

A new backlight binding is added to configure the backlight
on the secondary display. Also a new backlight type is added
for panels that have backlight controlled externally.

CRs-Fixed: 2363156
Change-Id: Ie5c610261635d0417c1afd9cd0f60f68a4a58c71
Signed-off-by: Satya Rama Aditya Pinapala <psraditya30@codeaurora.org>