MIPS: BCM63XX: Move DMA descriptor definition into common header file

The "IUDMA" engine used by bcm63xx_enet is also used by other blocks,
such as the USB 2.0 device.  Move the definitions into a common file so
that they do not need to be duplicated in each driver.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Patchwork: http://patchwork.linux-mips.org/patch/4082/
Signed-off-by: John Crispin <blogic@openwrt.org>