stmmac: add ipq806x glue layer

Hi Dan,
You're right, that's a bug. The offset should be (x), and not (1 << x). We could either redefine the macro to be (x), or we just use << gmac->id directly without a macro.
The reason why we have NSS_COMMON_CLK_SRC_CTRL_RGMII & NSS_COMMON_CLK_SRC_CTRL_SGMII macros is that the bit takes different values depending on the port. I can post a patch today if you want.