mx6cuboxi: Pull down PAD_ENET_RXD0/RXD1

Configure PAD_ENET_RXD0/RXD1 pads as pull down because these pads are directly
connected to the Atheros 8035/8030 although they should be functional
only in the RMII mode - 8030.

Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-By: Vagrant Cascadian <vagrant@aikidev.net>