ARM: 6239/1: mmci: let core poll for card detection

Use the MMC core's ability to poll for card detection.  This also has
the advantage of doing the gpio_get_value from a workqueue instead of
timer, allowing the gpio to be on a sleeping gpiochip.

Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>