clk: imx6: Add definition for IMX6QDL_CLK_ENET 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 IMX6QDL_CLK_ENET.

This change defines the missing clock in i.MX6Q's CCF.

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