Platform/RaspberryPi: Invert default Arasan, eMMC2 routing

In order for the WiFi to work, and the SD to run at full
speed we need to bind the SD slot to the eMMC2 controller.

Since we now have a driver for the eMMC2 controller
there isn't any reason to leave the SD card bound
to the older Arasan controller.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>