ARM: davinci_all_defconfig: set CONFIG_BACKLIGHT_PWM=m

The da850-evm came with an LCD with a backlight tied to a pwm. This
enables CONFIG_BACKLIGHT_PWM as a module so the PWM backlight can be
used to maintain brightness control of the backlight.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>