net: sun8i_emac: Retrieve GMAC clock via 'syscon' phandle

Unlike other Allwinner SoC's R40 GMAC clock control register
is locate in CCU, but rest located via syscon itself. Since
the phandle property for current code look for 'syscon' and
it will grab the respective ccu or syscon base address based
on DT property defined in respective SoC dtsi.

So, use the existing 'syscon' code even for R40 for retrieving
GMAC clock via CCU and update the register directly in
sun8i_emac_set_syscon instead of writing it separately using
ccm base.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Lothar Felten <lothar.felten@gmail.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>