mmc: omap_hsmmc: make it possible to compile out ADMA support

Some platforms don't have ADMA controllers. For those platforms, compiling
it out reduces the size of the binary by about 600 bytes.
Leaving the support in doesn't break things as the driver checks at runtime
if the ADMA2 controller is present.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Tested-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>