SPARC/LEON3: Added GRETH Ethernet 10/100/1000 driver.

GRETH is an Ethernet 10/100 or 10/100/1000 MAC with out without
a debug link (EDCL). The GRETH core is documented in GRIP.pdf
available at www.gaisler.com.

If the GRETH has GigaBit support (GBIT, Scatter gather, checksum
offloading etc.) can be determined by a bit in the control register.
The GBIT MAC is supported by operating in GRTEH 10/100 legacy mode.

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>