[ARM] 3487/1: IXP4xx: Support non-PCI systems

Patch from Deepak Saxena

This patch allows for the addition of IXP4xx systems that do not make
use of the PCI interface by moving the CONFIG_PCI symbol selection to
be platform-specific instead of for all of IXP4xx. If at least one machine
with PCI support is built, the PCI code will be compiled in, but when
building !PCI, this will drastically shrink the kernel size.

Signed-off-by: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>