arm64: set CONFIG_MMC_BCM2835=y in defconfig

We need to enable this controller so that we can switch the SD card's
pinmux over to it by default, which will improve storage performance.

Read access (dd with 64k blocks on rpi2):
  CONFIG_MMC_SDHCI_IPROC: 11-12 MB/s
  CONFIG_MMC_BCM2835:     19-20 MB/s

Differences on write access are pretty much in the noise.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>