GMAC: add document for Rockchip RK3288 GMAC

The document descripts how to add properties for GMAC in device tree.

change since v2:

1. remove power-gpio, reset-gpio, phyirq-gpio, pmu_regulator setting
2. add "snps,reset-gpio", "snps,reset-active-low;" "snps,reset-delays-us"

Signed-off-by: Roger Chen <roger.chen@rock-chips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>