tty: serial: remove pnx8xxx uart driver

Commit 625326ea ("MIPS: Remove PNX833x alias NXP_STB22x") removed
support for PNX833x, so it's time to remove serial driver, too.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Link: https://lore.kernel.org/r/20201106130332.103476-1-tsbogend@alpha.franken.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>