ARM: pxa: Initialize PWM backlight enable_gpio field

The GPIO API defines 0 as being a valid GPIO number, so this field needs
to be initialized explicitly.

A special case is the Palm Tungsten|C board. Since it doesn't use any
quirks that would require the existing .init() or .exit() hooks it can
simply use the new enable_gpio field.

Signed-off-by: Thierry Reding <treding@nvidia.com>