board: colibri-imx6ull: fix detecting ethernet phy

Now that it is possible to use regulator-fixed-clock make use
of it. This makes U-Boot detect the PHY on first cold-boot.

This commit also adjusts the code in setup_fec and follows
how it is done in mx6ullevk.c

This commit also slows down the boot-process by about 150ms
as it now waits for the regulator-fixed-clock voltage that
drives the PHY to go up.
If you rely on very fast boot-speeds and don't need ethernet
for your boot-process you can safely revert the changes on
imx6ull-colibri.dtsi

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>