backlight: Add Cirrus EP93xx backlight driver

The EP9307, EP9312, and EP9315 processors include a framebuffer
peripheral.  This peripheral has a dedicated pwm output called
BRIGHT that can be used to control the backlight on an LCD.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>