net: arc_emac: make the rockchip emac document more compatible

Add the rk3036 SoCs to match driver for document since the emac driver
has supported the rk3036 SoCs.

This patch adds the rk3036/rk3066/rk3188 SoCS to compatible for rockchip
emac ducument. Also, that will suit for other SoCs in the future.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: devicetree@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>