Probe the SEAD3 EHCI controller using the generic-ehci driver & device
tree rather than platform code, in order to reduce the amount of the
latter.

Now that no devices probed from platform code require interrupts, remove
the retrieval of the IRQ domain & sead3int.h.
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/14051/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>