net: sun8i_emac: Use consistent clock bitfield definitions

While the R40 uses a different register for EMAC clock configuration
than other chips, the register has a very similar layout. Reuse the
existing bitfield definitions in this file, since they match.

This allows the driver to compile on the H6 platform, where the
CCM_GMAC_CTRL definitions are not present.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>