of/address: merge of_iomap()

Merge common code between Microblaze and PowerPC.  This patch creates
new of_address.h and address.c files to containing address translation
and mapping routines.  First routine to be moved it of_iomap()

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Michal Simek <monstr@monstr.eu>
CC: Stephen Rothwell <sfr@canb.auug.org.au>