powerpc/44x: Add new compatible value for EMAC node of APM821XX dts file.

This compatible value will be used to distinguish some special features of APM821XX EMAC: no half duplex mode support, configuring jumbo frame.

Signed-off-by: Duc Dang <dhdang@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>