[ARM] 2936/1: ixp4xx default config fixes

Patch from Vincent Sanders

A recent patch which made IXP4xx mach_desc's depend on config options
had the effect of not building the kernel for several machines it
possibly could be, this patch updates the default config to ensure all
possible machines are built for by default.
Signed-off-by: Vincent Sanders <vince@arm.linux.org.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>