mx6slevk: Fix the power up of the Ethernet PHY

GPIO4_21 is the LAN8720 power pin, not the LAN8720 reset pin.

Fix that, so that we can have Ethernet functional again.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>