riscv: Prepare ptdump for vm layout dynamic addresses

This is a preparatory patch for sv48 support that will introduce
dynamic PAGE_OFFSET.

Dynamic PAGE_OFFSET implies that all zones (vmalloc, vmemmap, fixaddr...)
whose addresses depend on PAGE_OFFSET become dynamic and can't be used
to statically initialize the array used by ptdump to identify the
different zones of the vm layout.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>