MTD DocBook: fix ioremap return type

ioremap() returns a void __iomem * not an unsigned long.  Update the
Documentation file to reflect this.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>