stmmac: add ipq806x glue layer

The ethernet controller available in IPQ806x is a Synopsys DesignWare
Gigabit MAC IP core, already supported by the stmmac driver.

This glue layer implements some platform specific settings required to
get the controller working on an IPQ806x based platform.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>