net: gmac_rockchip: Add support for the RK3368 GMAC

net: gmac_rockchip: Add support for the RK3368 GMAC

The GMAC in the RK3368 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: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>