ACPI / processor: Add acpi_map_madt_entry()

Follow-on arm64 ACPI/NUMA patches need to map MADT entries very early
(before kmalloc is usable).

Add acpi_map_madt_entry() which, indirectly, uses
early_memremap()/early_memunmap() to access the table and parse out
the mpidr.  The existing implementation of map_madt_entry() is
modified to take a pointer to the MADT as a parameter and the callers
adjusted.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>