into a new function that other platforms can share.
This creates a new ofw_reg_to_paddr() function (in a new ofw_subr.c file)
that contains most of the existing ppc implementation, mostly unchanged.
The ppc code now calls the new MI code from the MD code, then creates a
ppc-specific bus_space mapping from the results. The new arm implementation
does the same in an arm-specific way.
This also moves the declaration of OF_decode_addr() from ofw_machdep.h to
openfirm.h, except on sparc64 which uses a different function signature.
This will help all FDT platforms to set up early console access using
OF_decode_addr().
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