powerpc: Move kdump default base address to half RMO size on 64bit

We are seeing boot failures on some very large boxes even with
commit b5416ca9 (powerpc: Move kdump default base address to
64MB on 64bit).

This patch halves the RMO so both kernels get about the same
amount of RMO memory. On large machines this region will be
at least 256MB, so each kernel will get 128MB.

We cap it at 256MB (small SLB size) since some early allocations need
to be in the bolted SLB region. We could relax this on machines with
1TB SLBs in a future patch.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>