[MIPS] rbtx4927: misc cleanups

* Merge tx4927_pci.h into tx4927.h
* Kill (broken) external PCI clock frequency reporting
* Kill unnecessary wbflush()
* Kill unnecessary includes
* Kill debug garbages

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>