arm64: kdump: Remove custom linux,usable-memory-range handling

Remove the architecture-specific code for handling the
"linux,usable-memory-range" property under the "/chosen" node in DT, as
the platform-agnostic FDT core code already takes care of this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/7356c531c49a24b4a55577bf8e46d93f4d8ae460.1628670468.git.geert+renesas@glider.be