visorbus: switch from ioremap_cache to memremap

In preparation for deprecating ioremap_cache() convert its usage in
visorbus to memremap.

Cc: Benjamin Romer <benjamin.romer@unisys.com>
Cc: David Kershner <david.kershner@unisys.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>