mmc: sdhci-iproc: Add support for emmc2 of the BCM2838

The emmc2 interface of the BCM2838 should be integrated in sdhci-iproc
to avoid code redundancy. Except 32 bit only access no other quirks are
known yet. Add an additional compatible string for upstream proposal.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>