usb: ehci-mx5: Add a default for CFG_MXC_USB_PORTSC

Just like drivers/usb/host/ehci-mx6.c, add a default for
drivers/usb/host/ehci-mx5.c.

The motivation for doing this is to remove CFG_MXC_USB_PORTSC
from board config files.

All the mx5 boards, with the exeption of mx51evk, define
CFG_MXC_USB_PORTSC as:

 #define CFG_MXC_USB_PORTSC	(PORT_PTS_UTMI | PORT_PTS_PTW)

So move this definition as a default into ehci-mx5.c.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Marek Vasut <marex@denx.de>