Handle dma mappings with more than one segment for rpi sdhci.

The driver inherently does dma in 512 byte chunks, but it's possible that
such a buffer can span two physically discontiguous pages (such as when
a userland program does IO on the raw /dev/mmcsdN devices). Now the driver
can handle a buffer that's split across two pages.

It could in theory handle any number of segments now, but as long as IO is
being done in 512 byte blocks it will never need more than two.