[MIPS] TXx9: Reorganize PCI code

Split out PCIC dependent code and SoC dependent code from board dependent
code.  Now TX4927 PCIC code is independent from TX4927/TX4938 SoC code.
Also fix some build problems on CONFIG_PCI=n.

As a bonus, "FPCIB0 Backplane Support" is available for all TX39/TX49 boards
and PCI66 support is available for all TX49 boards.

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