powerpc/xmon: add command to dump OPAL msglog

Add the 'do' command to dump the OPAL msglog in xmon.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
[mpe: Reduce the amount of ifdefery required]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>