net: gmac_rockchip: Add support for the RK3228 GMAC

net: gmac_rockchip: Add support for the RK3228 GMAC

The GMAC in the RK3228 once again is identical to the incarnation in
the RK3288 and the RK3399, except for where some of the configuration
and control registers are located in the GRF.

This adds the RK3368-specific logic necessary to reuse this driver.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>