MIPS: qemu-malta: bring up ethernet

Qemu emulates a PCNET PCI card for the Malta CoreLV board.
Enable the pcnet driver and add board specific ethernet
initialization function to bring it up. Also enable the
CONFIG_CMD_NET and CONFIG_CMD_PING options.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>