usb: gadget: Do not export usbd_device_* arrays

Each array is used only in one file (core.c or ep0.c). Move their content
to correct file, mark them as static and do not export out of current file.

This change allows to decrease size of u-boot.bin as more of those strings
are not used.

Signed-off-by: Pali Rohár <pali@kernel.org>