[ARM] pxamci: add simple gpio controls

The MMC block needs 3 external datas to work :
 - is the MMC card put in "read-only mode" ?
 - is a MMC card inserted or removed ?
 - enable power towards the MMC card

Several platforms provide these controls through
gpios. Expand the platform_data to request and use these
gpios is set up by board code.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Pierre.Ossman <pierre@ossman.eu>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>