target-arm: correct CNTFRQ access rights

target-arm: correct CNTFRQ access rights

Correct some corner cases we were getting wrong for
CNTFRQ access rights:
 * should UNDEF from 32-bit Secure EL1
 * only writable from the highest implemented exception level,
   which might not be EL1 now

To clarify the code, provide a new utility function
arm_highest_el() which returns the highest implemented
exception level.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>