powerpc/of_serial: Add missing ns16550a id

Many boards have a bug-free ns16550 compatible serial port, which we should
register as PORT_16550A. This introduces a new value "ns16550a" for the
compatible property of of_serial to let a firmware choose that model instead
of using the crippled PORT_16550 mode.

Reported-by: Alon Ziv <alonz@nolaviz.org>
Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>