powerpc/fadump: remove RMA_START and RMA_END macros

RMA_START is defined as '0' and there is even a BUILD_BUG_ON() to
make sure it is never anything else. Remove this macro and use '0'
instead as code change is needed anyway when it has to be something
else. Also, remove unused RMA_END macro.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821384096.5656.15026984053970204652.stgit@hbathini.in.ibm.com