[fdtmem] Record size of accessible physical address space

[fdtmem] Record size of accessible physical address space

The size of accessible physical address space will be required for the
runtime memory map, not just at relocation time.  Make this size an
additional parameter to fdt_register() (matching the prototype for
fdt_relocate()), and record the value for future reference.

Note that we cannot simply store the limit in fdt_relocate() since it
is called before .data is writable and before .bss is zeroed.

Signed-off-by: Michael Brown <mcb30@ipxe.org>