ARM: add CPU_THUMB_CAPABLE to indicate possible Thumb support

Clean up arch/arm/mm/Kconfig a little to provide a symbol which
indicates whether the CPU may support the Thumb instruction set.  This
gets rid of the growing dependencies on ARM_THUMB, and also gives us a
useful Kconfig symbol for choosing the kuser code.

Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>