dm: arm: bcmstb: Enable driver model MMC support

For bcm7445 and bcm7260, this patch enables CONFIG_DM_MMC and updates
the bcmstb SDHCI driver to use the new driver model.  This allows
removal of SDHCI configuration handling from bcmstb.c, and eliminates
a board removal compile warning.

Signed-off-by: Thomas Fitzsimmons <fitzsim@fitzsim.org>
Reviewed-by: Stefan Roese <sr@denx.de>