mmc: omap_hsmmc: Add support for DMA (ADMA2)

The omap hsmmc host controller can have the ADMA2 feature. It brings better
read and write throughput.
On most SOC, the capability is read from the hl_hwinfo register. On OMAP3,
DMA support is compiled out.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>