ARM: iWMMXt: always enable iWMMXt support with PJ4 CPUs

Jason Cooper reports these build errors:
arch/arm/kernel/built-in.o: In function `iwmmxt_do':
/.../arch/arm/kernel/pj4-cp0.c:36: undefined reference to `iwmmxt_task_release'
/.../arch/arm/kernel/pj4-cp0.c:40: undefined reference to `iwmmxt_task_switch'
make: *** [vmlinux] Error 1

This is caused because the PJ4 code explicitly references the iWMMXt
code, but doesn't require it to be built.  Fix this by ensuring that
iWMMXt is always enabled with PJ4.

Reported-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>