colibri_imx6: set gpr1 ENET_CLK_SEL

This fixes the issue when PHY auto negotiation never completes.

Fixes: 431cd76d("colibri_imx6: migrate to DM_ETH")
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>