mw_debug: Add support for reading GSPRs and writing memory

mw_debug: Add support for reading GSPRs and writing memory

This adds a "gpr" command for reading 1 or more GPRs/fast SPRs,
and a "mw" command for writing an 8-byte value to memory.  It also
adds an "icreset" command for resetting the instruction cache
and fixes the "creset" command to actually reset the core instead
of starting it.  The MSR is now printed along with the NIA in the
status information.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>