powerpc/xmon: Dump XIVE information for online-only processors.

dxa command in XMON debugger iterates through all possible processors.
As a result, empty lines are printed even for processors which are not
online.

CPU 47:pp=00 CPPR=ff IPI=0x0040002f PQ=-- EQ idx=699 T=0 00000000 00000000
CPU 48:
CPU 49:

Restrict XIVE information(dxa) to be displayed for online processors only.

Signed-off-by: Sachin Sant <sachinp@linux.vnet.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/164139226833.12930.272224382183014664.sendpatchset@MacBook-Pro.local