Add support for parsing the "linux,usable-memory-range" DT property.
This property is used to describe the usable memory reserved for the
crash dump kernel, and thus makes the memory reservation explicit.
If present, Linux no longer needs to mask the program counter, and rely
on the "mem=" kernel parameter to obtain the start and size of usable
memory.

For backwards compatibility, the traditional method to derive the start
of memory is still used if "linux,usable-memory-range" is absent.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>