mtd: pxa2xx-flash: switch from ioremap_cache to memremap

In preparation for deprecating ioremap_cache() convert its usage in
pxa2xx-flash to memremap.

Cc: David Woodhouse <dwmw2@infradead.org>
[brian: also convert iounmap to memunmap]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>