net: stmmac: add Amlogic Meson glue layer

The Ethernet controller available in Meson6 and Meson8 SoCs is a
Synopsys DesignWare MAC IP core, already supported by the stmmac
driver.

This glue layer implements some platform-specific settings needed by
the Amlogic variant.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>