target-arm: Remove failure status return from read/write_raw_cp_reg

target-arm: Remove failure status return from read/write_raw_cp_reg

The read_raw_cp_reg and write_raw_cp_reg functions can now never
fail (in fact they should never have failed previously unless
there was a bug in a reginfo that meant no raw accessor was
provided for a might-trap register). This allows us to clean up
their prototypes so the write function returns void and the
read function returns the value read, which in turn lets us
simplify the callers.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>