mmc: jz4740: Use GPIO descriptor for power

The power GPIO line is passed with inversion flags and all from
the platform data. Switch to using an optional GPIO descriptor and
use this to switch the power.

Augment the only boardfile to pass in the proper "power" descriptor
in the GPIO descriptor machine table instead.

As the GPIO handling is now much simpler, we can cut down on some
overhead code.

Cc: Paul Cercueil <paul@crapouillou.net>
Cc: linux-mips@linux-mips.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>