avr32: use GPIO line PB15 on EVKLCD10x boards for backlight

The PB15 GPIO line is used to control the enable and disable signal for
the backlight regulator on EVKLCD10x boards. This patch hands the I/O
line over to the LCDC driver, which will control when to enable and
disable the backlight.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
[haavard.skinnemoen@atmel.com: reverted ac97c change]
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>