pm80xx: wait a minimum of 500ms before issuing commands to SPCv

The documentation for the 8070 and 8072 SPCv chip explicitly states that
a minimum of 500ms must elapse before issuing commands, otherwise the
SPCv may not process them and the firmware may get into an unrecoverable
state requiring a reboot.  While the Linux guys will probably think this
is 'racy', it is called out in the chip documentation and inserting this
delay makes power management function properly.

Signed-off-by: Benjamin Rood <brood@attotech.com>
Reviewed-by: Jack Wang <jinpu.wang@profitbricks.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>