target-arm: Fix incorrect type for value argument to write_raw_cp_reg

target-arm: Fix incorrect type for value argument to write_raw_cp_reg

The write_raw_cp_reg's value argument should be a uint64_t, since
that's what all its callers hand it and what all the functions it
calls take. A (harmless) typo meant we were accidentally declaring
it as int64_t.

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