SAMA5D4: Initial XDMAC driver logic; initial check-in is little more the the DMAC driver with some name changes

SAMA5D3/4:  More renaming.  Change SAMA5D3 EMAC to EMACA and SAMA5D4 to EMACB so that the configuration and build system can configure them.  I might come up with something better later

SAMA5:  Back out most of commit c37b5b7b97d0644743c04f2c3d9e2b7ef9f5d698.  Things are going to have to be done differently