dt: bcm2835: add mmc node

Add MMC node, increase clk_mmc frequency and use bcm2835-mmc instead of
bcm2835-sdhci

Based on Noralf Tronnes code
https://github.com/notro/rpi-bcm2835/blob/3af7ed5b12d436829c4286baab94e68c93b2319d/Rakefile

Signed-off-by: Piotr Król <piotr.krol@3mdeb.com>