Platform/RaspberryPi: User control of eMMC2 DMA

DMA translation on the eMMC2 vary based on SoC, and
this is made worse by the poor _DMA support in Linux.

For now the "safe" option is to simply run the eMMC2
controller in PIO mode. More advanced users or !Linux
operating systems may choose to enable this to gain
a perf boost.

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