net: stmmac: dwmac_socfpga: This platform has GMAC

net: stmmac: dwmac_socfpga: This platform has GMAC

Indicate that dwmac_socfpga has a gmac. This will make sure that
gmac-specific interrupt processing is done, including timestamp
interrupt handling. Without this, the external snapshot interrupt is
never ack'd and we have an interrupt storm on external snapshot event.

Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20241112170658.2388529-10-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>