ARM: pxa: Add dummy backlight power supply on Mitac Mio A701

Recent changes to the pwm-backlight driver have made the power supply
mandatory. There is code in the regulator core to deal with situations
where no regulator is specified and provide a dummy, but that works on
DT-based boards only.

The situation can be remedied by adding a dummy regulator during board
initialization.

Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>