All cpreg read and write functions now return 0, so we can clean up
their prototypes:
 * write functions return void
 * read functions return the value rather than taking a pointer
   to write the value to

This is a fairly mechanical change which makes only the bare
minimum set of changes to the callers of read and write functions.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>