backlight: lp855x: add FAST bit description for LP8556

LP8556 backlight driver supports fast refresh mode when exiting the low
power mode.  This bit can be configurable in the platform side.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>