ARM: imx6q: Set enet tx reference clk from anatop to support 1588

Set GRP1 BIT21 ENET_CLK_SEL:
  Enet tx reference clk from internal clock from anatop
  (loopback through pad), this clock also sent out to external PHY

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>