dm: mmc: rpi: Convert Raspberry Pi to driver model for MMC

Convert the bcm2835 SDHCI driver over to support CONFIG_DM_MMC and move
all boards over. There is no need to keep the old code since there are no
other users.

Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Simon Glass <sjg@chromium.org>