powerpc/8xx: Rework CONFIG_PIN_TLB handling

On recent kernels, with some debug options like for instance
CONFIG_LOCKDEP, the BSS requires more than 8M memory, allthough
the kernel code fits in the first 8M.
Today, it is necessary to activate CONFIG_PIN_TLB to get more than 8M
at startup, allthough pinning TLB is not necessary for that.

We could have inconditionaly mapped 16 or 24M bytes at startup
but some old hardware only have 8M and mapping non-existing RAM
would be an issue due to speculative accesses.

With the preceding patch however, the TLB entries are populated on
demand. By setting up the TLB miss handler to handle up to 24M until
the handler is patched for the entire memory space, it is possible
to allow access up to more memory without mapping non-existing RAM.

It is therefore not needed anymore to map memory data at all
at startup. It will be handled by the TLB miss handler.

One might still want to PIN the IMMR and the first 24M of RAM.
It is now possible to do it in the C memory initialisation
functions. In addition, we now know how much memory we have
when we do it, so we are able to adapt the pining to the
real amount of memory available. So boards with less than 24M
can now also benefit from PIN_TLB.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <oss@buserror.net>