soc/cores: add usb_fifo with FT245 USB FIFO PHY from LiteUSB, deprecate LiteUSB

soc/cores: add usb_fifo with FT245 USB FIFO PHY from LiteUSB, deprecate LiteUSB

LiteUSB was not up to date was not a real USB PHY but was just providing USB FIFO PHYs.
New true USB cores are now available: Daisho, ValentyUSB, so it's better using
then for true USB support. We only keep the FT245 FIFO PHY in LiteX that can be
useful to interface with USB2/USB3 USB FIFOs.