[ARM] nommu: prevent Xscale-based machines being selected

Hyok says Intel Xscale is not currently supported by uCLinux.
Rather than adding #error statements to the Xscale support files
and causing !MMU+Xscale configurations to fail to build, prevent
Xscale-based machines from being selected in !MMU mode.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>