[POWERPC] add of_iomap function

The of_iomap function maps memory for a given
device_node and returns a pointer to that memory.
This is used at some places, so it makes sense to
a seperate function.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>