target-arm: Allow cp access functions to indicate traps to EL2 or EL3

target-arm: Allow cp access functions to indicate traps to EL2 or EL3

Some coprocessor access functions will need to indicate that the
instruction should trap to EL2 or EL3 rather than the default
target exception level; add corresponding CPAccessResult enum
entries and handling code.

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