net: sun8i-emac: support R40 GMAC

Add support for the GMAC found in the Allwinner R40/V40 SoC.

The R40 GMAC interface is not controlled by the syscon register but
has a separate configuration register in the CCU.
The clock gate and reset bits are in a different register compared
to the other SoCs supported by this driver.
The driver uses the -gmac suffix for the R40 because the R40 also
has a different 100 MBit MAC (EMAC).

Signed-off-by: Lothar Felten <lothar.felten@gmail.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@openedev.com>