mach-u300: use mmci driver for GPIO card detect

The mmci driver can handle a GPIO pin for card detect, using
IRQs and all just fine, so switch to using that. Delete the
old bogus input device hack, if userspace need to detect
MMC cards it should use udev like everyone else.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>