MIPS: Malta: Probe RTC via DT

Add the DT node required to probe the RTC, and remove the platform code
that was previously doing it.

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/14277/


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