MAINTAINERS: Add LAN78XX entry

Add maintainers for Microchip LAN78XX.
UNGLinuxDriver@microchip.com is alias email which goes to current
developers work for Microchip Network related products.

Signed-off-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>