mmc: pxamci: call mmc_of_parse()

Call into mmc_of_parse() from pxamci_of_init(). As it needs a pointer to a
struct mmc_host, refactor the code a bit.

This allows all generic MMC properties to be set that are described in
Documentation/devicetree/bindings/mmc/mmc.txt. Reword the documentation
a bit to make that clear.

The "cd" and "wp" gpio lookups are removed as the lookup will now be
done by mmc_of_parse().

Signed-off-by: Daniel Mack <daniel@zonque.org>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>