net: stmmac: Do not cut down 1G modes

Some glue logic drivers support 1G without having GMAC/GMAC4/XGMAC.

Let's allow this speed by default.

Reported-by: Ondrej Jirman <megi@xff.cz>
Tested-by: Ondrej Jirman <megi@xff.cz>
Fixes: 5b0d7d7d ("net: stmmac: Add the missing speeds that XGMAC supports")
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>