The Ethernet refclock configuration is board specific and should
not be harcoded in machine code.

Remove it to align with the imx6ul commit e87f3be1 ("ARM: mach-imx:
imx6ul: remove not optional ethernet refclock overwrite").

Clearing bits 13 and 17 of GPR1 is the POR values, so this change
does not affect existing boards in mainline.

Tested on imx6sx-udoo-neo and imx6sx-sdb boards.
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>