MIPS: Malta: Probe interrupt controllers via DT

Probe the CPU, GIC & i8259 interrupt controllers present in the Malta
system using device tree. This enables interrupts to be provided to
devices using device tree as they are moved over to being probed using
it.

Since Malta is very configurable it's unknown whether a GIC will be
present at compile time. In order to support both cases the
malta_dt_shim code is added in order to detect whether a GIC is present,
adjusting the DT to route interrupts correctly and nop out the GIC node
if no GIC is found.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14274/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -479,6 +479,7 @@ config MIPS_MALTA
 	select SYS_SUPPORTS_ZBOOT
 	select SYS_SUPPORTS_RELOCATABLE
 	select USE_OF
+	select LIBFDT
 	select ZONE_DMA32 if 64BIT
 	select BUILTIN_DTB
 	select LIBFDT