Move pxamci to mmc slot-gpio API to fix interrupt request.

It fixes the case where the card detection is on a gpio expander, on I2C
for example on zylonite board. In this case, the card detect netsted
interrupt is called from a threaded interrupt. The request_irq() fails,
because a hard irq cannot be a nested interrupt from a threaded
interrupt (set __setup_irq()).

This was tested on zylonite and mioa701 boards.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Petr Cvek <petr.cvek@tul.cz>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>