target-arm: Allow special cpregs to have flags set

Relax the "is this a valid ARMCPRegInfo type value?" check to permit
"special" cpregs to have flags other than ARM_CP_SPECIAL set. At
the moment none of the other flags are relevant for special regs,
but the migration related flag we're about to introduce can apply
here too.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
