malta: enable CONFIG_PCNET_79C973, PCNET_HAS_PROM, CONFIG_CMD_DHCP

This model of the pcnet is used in current Malta boards, at least in the
Malta-R rev 3. Enable support for it.

The Malta also has the ethernet controller PROM containing its MAC
address, so enable support for that in order to read that MAC address.

DHCP is a very useful feature to have available for many networks,
enable support for it also.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>