at91: add USB kmods only for subtargets that support USB

at91: add USB kmods only for subtargets that support USB

Currently, kmod-usb-ohci,kmod-at91-udc and kmod-usb-gadget-eth are included
as the default packages for all at91 subtargets.

However, this is breaking image builder as kmod-at91-udc is not being built
on sama7 since it depends on USB_SUPPORT and sama7 does not have USB support
enabled in the kernel as its not supported upstream so its not even
selectable in the config.

So, move to include these as default packages only for sama5 and sama9x as
both of those have USB support enabled.

Fixes: #18407
Link: https://github.com/openwrt/openwrt/pull/18413
Signed-off-by: Robert Marko <robimarko@gmail.com>