[ARM] 3612/1: make pci bus optional for ixp4xx platform

Patch from Milan Svoboda

IXP4XX platform can happily live without pci bus. This patch modifies
Kconfig to support this option and modifies Makefile so pci only files
are compiled only when pci is really selected.

Patch is tested and ixdp465 runs fine with or without the pci bus.--

Signed-off-by: Milan Svoboda &lt;msvoboda@ra.rockwell.com&gt;Signed-off-by: Deepak Saxena &lt;dsaxena@plexity.net&gt;Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>