net/macb: ethtool interface: add register dump feature

Add macb_get_regs() ethtool function and its helper function:
macb_get_regs_len().
The version field is deduced from the IP revision which gives the
"MACB or GEM" information. An additional version field is reserved.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Tested-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>