mos6522: fix linking error when CONFIG_MOS6522 is not set

When CONFIG_MOS6522 is not set, building ppc64-softmmu target fails:

/usr/bin/ld: libqemu-ppc64-softmmu.fa.p/monitor_misc.c.o:(.data+0x1158): undefined reference to `hmp_info_via'

Make devices configuration available in hmp-commands*.hx and check for
CONFIG_MOS6522.

Fixes: 409e9f71 (mos6522: add "info via" HMP command for debugging)
Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Cc: Fabiano Rosas <farosas@linux.ibm.com>
Cc: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220510235439.54775-1-muriloo@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>