usb: ehci-mx6: Set default CONFIG_MXC_USB_PORTSC if not defined

usb: ehci-mx6: Set default CONFIG_MXC_USB_PORTSC if not defined

There is now multiple copies of CONFIG_MXC_USB_PORTSC in configs set to
PORT_PTS_UTMI | PORT_PTS_PTW, which is in fact the default register value
for MX6, MX7 and MX7ULP. Define the default value of CONFIG_MXC_USB_PORTSC
in the driver and use it in case CONFIG_MXC_USB_PORTSC is not defined in
config, to reduce the duplication.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>