emac: Provide generic function to get an MDH EMAC instance

Rename `lwip_emac_get_default_instance` to
`mdh_emac_get_default_instance` so it can be called
by other network stacks that require an MCU-Driver-HAL
EMAC instance.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>