ARM: imx: mach-imx6q: Fix the PHY ID mask for AR8031

AR8031 and AR8035 have the same PHY ID mask of 0xffffffef.

So fix it and make it match with the PHY ID mask definition
at drivers/net/phy/at803x.c.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>