ARM: pxa: always select one of the two CPU types

When all boards are disabled on PXA, we cannot build a kernel
because no CPU gets selected:

arch/arm/include/uapi/asm/swab.h:26:29: error: "__LINUX_ARM_ARCH__" is not defined [-Werror=undef]

This is a bit annoying for compile-testing, so I'm adding a line
that ensures that at all times, at least one of CPU_XSCALE or
CPU_XSC3 is set and we can at least continue building.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>