eeprom: SiFive Unmatched: re-implement mac command

The different implementations of the mac command have board or vendor
specific sub-commands.

Add the command definition specific to the SiFive HiFive Unmatched board.

Don't call cmd_usage() directly but return CMD_RET_USAGE instead.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>