clk: imx6: Add definition for IMX6QDL_CLK_ENET_REF clock

After commit 673f6597 ("net: fec_mxc: support i.MX8M with CLK_CCF") all
NXP boards, which are not IMX8 and in the same time are supporting CCF need
to provide PTP clock.

On the i.MX6Q this clock is provided with IMX6QDL_CLK_ENET_REF in the Linux
kernel's CCF.

Code in this change models the simplest case when enet reference clock is
generated from 'osc' clock.

Signed-off-by: Lukasz Majewski <lukma@denx.de>