kdump: Allow shrinking of kdump region to be overridden

On ppc64 the crashkernel region almost always overlaps an area of firmware.
This works fine except when using the sysfs interface to reduce the kdump
region. If we free the firmware area we are guaranteed to crash.

Rename free_reserved_phys_range to crash_free_reserved_phys_range and make
it a weak function so we can override it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>