[ARM] 2917/1: Make IXP4xx mach_desc's depend on config options

Patch from Deepak Saxena

Building a kernel for IXDP425 currently includes the machine descriptors
for IXDP465 and PRPMC1100 even if those machines are not configured.
This means we can build a kernel that boots on those machines even
though the machine_is_xxx() macro will always return 0 and other bits
such as PCI won't be compiled in. This can lead to many wasted hours
wondering what you have done to your kernel to make it randomly crash
thus requireing large quantities of beer to be consumed. While I am
all for consumption of large quantities of beer, there are better
reasons to do so then stupid kernel bugs.

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