ARM: multi_v7_defconfig: Switch BCM2835 to sdhci-iproc.c for MMC

This approximately triples write performance for the SD card.  My card
is too full of important data to collect very reliable numbers, but I
see 271.361% +/- 166.742% improvement (n=3 before, 6 after), for 'dd
if=/dev/zero of=/boot/asdf bs=1M count=3 oflag=dsync,direct'.  Read
performance appears to be unaffected.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>