imx: Remove CFG_MXC_USB_PORTSC

The definition of CFG_MXC_USB_PORTSC as (PORT_PTS_UTMI | PORT_PTS_PTW)
can be removed from mx5/mx6/mx7/mx8m board config files as it is the
default in drivers/usb/host/ehci-mx5.c and drivers/usb/host/ehci-mx6.c.

Suggested-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Tim Harvey <tharvey@gateworks.com>