fbdev: sh_mobile_lcdcfb: add backlight support

Support for backlight devices controlled through board-specific
routines. Backlights can be defined per-channel and follow fbdev
directives to switch off as the LCD blanks or is turned on/off.

Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>