net: stmmac: rk3588: Allow multiple gmac controller

RK3588(s) can have multiple gmac controllers.
Re-use rk3568 logic to distinguish them.

Fixes: 2f2b60a0

 ("net: ethernet: stmmac: dwmac-rk: Add gmac support for rk3588")
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20221021172422.88534-1-sebastian.reichel@collabora.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>