mmc: sdhci-dove: allow GPIOs to be used for card detection on Dove

This commit taken from Rabeeh's Cubox kernel and re-worked for DT;
Sebastian Hasselbrath is believed to be the original author.

Some Cuboxes require a GPIO for card detection; this implements the
optional GPIO support for card detection.  This GPIO is logic 0 for
card inserted.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Chris Ball <cjb@laptop.org>