On imx6qdl, the ENET RMII and PTP clock can come from either internal
ANATOP/CCM or external clock source through pad GPIO_16.  But in case
of the external clock source, bit IOMUXC_GPR1[21] needs to be cleared.

The patch adds the support for systems that use an external clock source
and distinguishes above two cases by checking if the PTP clock specified
in device tree is the one coming from the internal ANATOP/CCM.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>