ieee1275: Support runtime memory claiming

ieee1275: Support runtime memory claiming

On powerpc-ieee1275, we are running out of memory trying to verify
anything. This is because:

 - we have to load an entire file into memory to verify it. This is
   difficult to change with appended signatures.
 - We only have 32MB of heap.
 - Distro kernels are now often around 30MB.

So we want to be able to claim more memory from OpenFirmware for our heap
at runtime.

There are some complications:

 - The grub mm code isn't the only thing that will make claims on
   memory from OpenFirmware:

    * PFW/SLOF will have claimed some for their own use.

    * The ieee1275 loader will try to find other bits of memory that we
      haven't claimed to place the kernel and initrd when we go to boot.

    * Once we load Linux, it will also try to claim memory. It claims
      memory without any reference to /memory/available, it just starts
      at min(top of RMO, 768MB) and works down. So we need to avoid this
      area. See arch/powerpc/kernel/prom_init.c as of v5.11.

 - The smallest amount of memory a ppc64 KVM guest can have is 256MB.
   It doesn't work with distro kernels but can work with custom kernels.
   We should maintain support for that. (ppc32 can boot with even less,
   and we shouldn't break that either.)

 - Even if a VM has more memory, the memory OpenFirmware makes available
   as Real Memory Area can be restricted. Even with our CAS work, an LPAR
   on a PowerVM box is likely to have only 512MB available to OpenFirmware
   even if it has many gigabytes of memory allocated.

What should we do?

We don't know in advance how big the kernel and initrd are going to be,
which makes figuring out how much memory we can take a bit tricky.

To figure out how much memory we should leave unused, I looked at:

 - an Ubuntu 20.04.1 ppc64le pseries KVM guest:
    vmlinux: ~30MB
    initrd:  ~50MB

 - a RHEL8.2 ppc64le pseries KVM guest:
    vmlinux: ~30MB
    initrd:  ~30MB

So to give us a little wriggle room, I think we want to leave at least
128MB for the loader to put vmlinux and initrd in memory and leave Linux
with space to satisfy its early allocations.

Allow other space to be allocated at runtime.

Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Tested-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Reviewed-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>