MIPS: SEAD3: Reset via generic syscon-reboot driver & DT

Remove the SEAD3 implementation of _machine_restart & instead make use
of the generic syscon-reboot driver probed via device tree.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14056/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>