Define ppc_md.set_dabr for both 32 + 64 bit. Cleanup the implementation for
pSeries also, it was needlessly complex. Now we just do two firmware tests at
setup time, and use one of two functions, rather than using one function and
testing on every call.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>