ARM: vexpress: allow dcscb and tc2_pm in a combined ARMv6+v7 build

This fixes the following build error:

/tmp/cce439dZ.s: Assembler messages:
/tmp/cce439dZ.s:506: Error: selected processor does not support ARM mode `isb '
/tmp/cce439dZ.s:512: Error: selected processor does not support ARM mode `isb '
/tmp/cce439dZ.s:513: Error: selected processor does not support ARM mode `dsb '
/tmp/cce439dZ.s:583: Error: selected processor does not support ARM mode `isb '
/tmp/cce439dZ.s:589: Error: selected processor does not support ARM mode `isb '
/tmp/cce439dZ.s:590: Error: selected processor does not support ARM mode `dsb '

Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>