cores/uart/UART: add stream interface (phy=None), add connect method and use this...

cores/uart/UART: add stream interface (phy=None), add connect method and use this for UART Stub/Crossover.

A bridged/crossover UART can now just be created by:
- passing uart_name="stream" to SoCCore/SoCSDRAM.
- adding a crossover UART core to the design:

# UART Crossover (over Wishbone Bridge
from litex.soc.cores.uart import UART
self.submodules.uart_xover = UART(tx_fifo_depth=2, rx_fifo_depth=2)
self.add_csr("uart_xover")
self.comb += self.uart.connect(self.uart_xover)