Revert "powerpc: Sync RPA note in zImage with kernel's RPA note"

This reverts commit 91a00302, plus
commit 0dcd4401 ("powerpc: Revert CHRP
boot wrapper to real-base = 12MB on 32-bit") which depended on it.

Commit 91a00302

 was causing NVRAM corruption on some pSeries machines,
for as-yet unknown reasons, so this reverts it until the cause is
identified.

Signed-off-by: Paul Mackerras <paulus@samba.org>