net: stmmac: Add glue layer for Loongson-1 SoC

This glue driver is created based on the arch-code
implemented earlier with the platform-specific settings.

Use syscon for SYSCON register access.

And modify MAINTAINERS to add a new F: entry for this driver.

Partially based on the previous work by Serge Semin.

Signed-off-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>