uart: Import and hook up opencore 16550 compatible UART

uart: Import and hook up opencore 16550 compatible UART

This imports via fusesoc a 16550 compatible (ie "standard") UART,
and wires it up optionally in the SoC instead of the potato one.

This also adds support for a second UART (which is always a
16550) to Arty, wired to JC "bottom" port.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>