MIPS: Add support for the IMG Pistachio SoC

MIPS: Add support for the IMG Pistachio SoC

Add initial support for boards based on the Imagination Pistachio SoC.
Pistachio is based on a dual-core MIPS interAptiv CPU and will boot
using device-tree.

Signed-off-by: James Hartley <james.hartley@imgtec.com>
Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/9569/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>