MLK-10098 ARM: imx: fix 1588 clock init

The enet clock define is changed as there has no "enet_ref" clock name.
If the tx_clk is sourced from SOC anatop PLL, user define the clock id
in devicetree. So we only to judge the ptp clock valid and then set the
related GPR bit.

Signed-off-by: Fugang Duan <B38611@freescale.com>