include/configs: replace MXC_EHCI_MODE_SERIAL with PORT_PTS_SERIAL

The MXC_EHCI_MODE_ definitions are redundant. Replace MXC_EHCI_MODE_SERIAL
with the equivalent PORT_PTS_SERIAL.

Only the zmx25 platform is affected.

Signed-off-by: Markus Niebel <Markus.Niebel@ew.tq-group.com>
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>