mmc: pxa: Use GPIO descriptor for power

After converting the PXA driver to use GPIO descriptors for
card detect and write protect it is relatively simple to
convert it to also use a descriptor for getting the optional
power control GPIO.

The polarity inversion flag can also go away from the platform
data since this is indicated in the GPIO machine descriptor
table.

Cc: Daniel Mack <daniel@zonque.org>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Andrea Adami <andrea.adami@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>