mmc: pxamci: let mmc core handle regulators

Strip some code by letting the mmc core handle the regulators. The old
.gpio_power pdata handling is kept around for now.

This also set the voltage on the regulator and handles -EPROBE_DEFER
correctly.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>