mmc: mmci: Use correct GPIO binding for IRQ requests

Now there are irqdomains in place for Snowball, we can request GPIO
IRQs directly by their binding. This replaces the previous method
of hard-coding the hwirq using u32 values in the DT.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>