MIPS: PNX8550: Move code one directory level up.

It was sharing the nxp directory but no code with pnx833x and will fit
better into the new platform makefile scheme, if moved.  Also after the
pnx833x code has been moved up, the pnx8550 Code was the last users of
the nxp dir.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>