Documentation: fix ioremap return type

ioremap() returns a void __iomem * not a char *.  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: Linus Torvalds <torvalds@linux-foundation.org>