powerpc/eeh: Introduce eeh_for_each_pe()

Add a for_each-style macro for iterating through PEs without the
boilerplate required by a traversal function. eeh_pe_next() is now
exported, as it is now used directly in place.

Signed-off-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>