device-tree: spi: make spi-bcm2835 the default spi driver and prepare for dma

* make spi-bcm2835 the default driver for spi
* add a fallback spi-bcm2708 overlay
* add dma entries to device tree for future updates
* add default cs-gpios entry showing how to extend the number of chip-selects.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>