imx: Remove CFG_MXC_USB_FLAGS

CFG_MXC_USB_FLAGS is only used for drivers/usb/host/ehci-mx5.c, so it
can be removed from all the imx6/imx7/imx8m board config files.

mx51evk.h is the only place CFG_MXC_USB_FLAGS is not set to 0.

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