net: stmmac: Add Adaptrum Anarion GMAC glue layer

Before the GMAC on the Anarion chip can be used, the PHY interface
selection must be configured with the DWMAC block in reset.

This layer covers a block containing only two registers. Although it
is possible to model this as a reset controller and use the "resets"
property of stmmac, it's much more intuitive to include this in the
glue layer instead.

At this time only RGMII is supported, because it is the only mode
which has been validated hardware-wise.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Signed-off-by: David S. Miller <davem@davemloft.net>