bcm2835-dma: add support for slave_sg transfer mode

Based on Gellert Weisz <gellert@raspberrypi.org> patch.

Signed-off-by: Piotr Król <piotr.krol@3mdeb.com>
bcm2835_defconfig: enable /proc/config.gz
bcm2835_defconfig: replace sdhci-bcm2835 with bcm2835-mmc and enable DMA