ARM: dts: AM33XX: Add MMC support and documentation

Adds AM33XX MMC support for am335x-bone, am335x-evm and am335x-evmsk boards.

Also added is the DMA binding definitions based on the generic DMA request
binding.

Additional changes made to DTS:
* Interrupt, reg and compatible properties added
* ti,needs-special-hs-handling added

Signed-off-by: Matt Porter <mporter@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>