powerpc: Revert CHRP boot wrapper to real-base = 12MB on 32-bit

Commit 9b09c6d9

 ("powerpc: Change the
default link address for pSeries zImage kernels") changed the
real-base value in the CHRP note added by addnote to the zImage from
12MB to 32MB.  It turns out that this causes unnecessary extra reboots
on old 32-bit CHRP machines.  This therefore adds a -r flag to addnote
to allow us to specify what real-base value it should put in the CHRP
note, and adjusts the wrapper script to pass -r c00000 to addnote when
making a zImage for a CHRP machine.  Also, CHRP machines ignore the
RPA note, so we don't need to arrange for it to be the same as the
kernel's.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>