MIPS: SEAD3: Parse memsize in DT shim

Parse the memsize argument provided by the bootloader in the DT shim
code, allowing the user to override it on the command line. This places
all of the DT manipulation code into sead3-dtshim.c.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14058/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>