net: remove cdns,at32ap7000-macb device tree entry

The AVR32 architecture has been removed from the kernel in commit
26202873

, hence clean out the
cdns,at32ap7000-macb compatible entry in Cadence macb Ethernet driver.

AVR32 architecture never supported device tree, hence this code was not
used by anybody.

Updated documentation to match the default entry, no users of
cdns,at32ap7000-macb in the kernel tree.

Signed-off-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>