of/address: Add new API of_iomap_by_name and of_get_address_by_name

Create new API of_iomap_by_name and of_get_address_by_name, which
would read "reg-names" property and get the reg address by its
name.

Change-Id: I7ce30da5a7cbc29f8349641af97ac7ed16b4f506
Signed-off-by: Xiaocheng Li <lix@codeaurora.org>
[imaund@codeaurora.org: Resolved trivial context conflicts.]
Signed-off-by: Ian Maund <imaund@codeaurora.org>