powerpc/serial: Use saner flags when creating legacy ports

powerpc/serial: Use saner flags when creating legacy ports

We had a mix & match of flags used when creating legacy ports
depending on where we found them in the device-tree. Among others
we were missing UPF_SKIP_TEST for some kind of ISA ports which is
a problem as quite a few UARTs out there don't support the loopback
test (such as a lot of BMCs).

Let's pick the set of flags used by the SoC code and generalize it
which means autoconf, no loopback test, irq maybe shared and fixed
port.

Sending to stable as the lack of UPF_SKIP_TEST is breaking
serial on some machines so I want this back into distros

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