powerpc/boot: Add a boot wrapper for Microwatt

This allows microwatt's kernel to be built with an embedded device tree.

Load to arch/powerpc/boot/dtbImage.microwatt to 0x500000:

 mw_debug -b fpga stop load arch/powerpc/boot/dtbImage.microwatt 500000 start

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/YMwX19wym3kQ7guu@thinks.paulus.ozlabs.org

arch/powerpc/boot/Makefile

@@ -163,6 +163,8 @@ src-plat-$(CONFIG_PPC_POWERNV) += pseries-head.S
 src-plat-$(CONFIG_PPC_IBM_CELL_BLADE) += pseries-head.S
 src-plat-$(CONFIG_MVME7100) += motload-head.S mvme7100.c
 
+src-plat-$(CONFIG_PPC_MICROWATT) += fixed-head.S microwatt.c
+
 src-wlib := $(sort $(src-wlib-y))
 src-plat := $(sort $(src-plat-y))
 src-boot := $(src-wlib) $(src-plat) empty.c
@@ -355,6 +357,8 @@ image-$(CONFIG_MVME5100)		+= dtbImage.mvme5100
 # Board port in arch/powerpc/platform/amigaone/Kconfig
 image-$(CONFIG_AMIGAONE)		+= cuImage.amigaone
 
+image-$(CONFIG_PPC_MICROWATT)		+= dtbImage.microwatt
+
 # For 32-bit powermacs, build the COFF and miboot images
 # as well as the ELF images.
 ifdef CONFIG_PPC32

arch/powerpc/boot/microwatt.c

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <stddef.h>
+#include "stdio.h"
+#include "types.h"
+#include "io.h"
+#include "ops.h"
+
+BSS_STACK(8192);
+
+void platform_init(unsigned long r3, unsigned long r4, unsigned long r5)
+{
+	unsigned long heapsize = 16*1024*1024 - (unsigned long)_end;
+
+	/*
+	 * Disable interrupts and turn off MSR_RI, since we'll
+	 * shortly be overwriting the interrupt vectors.
+	 */
+	__asm__ volatile("mtmsrd %0,1" : : "r" (0));
+
+	simple_alloc_init(_end, heapsize, 32, 64);
+	fdt_init(_dtb_start);
+	serial_console_init();
+}

arch/powerpc/boot/wrapper

@@ -342,6 +342,11 @@ gamecube|wii)
     link_address='0x600000'
     platformo="$object/$platform-head.o $object/$platform.o"
     ;;
+microwatt)
+    link_address='0x500000'
+    platformo="$object/fixed-head.o $object/$platform.o"
+    binary=y
+    ;;
 treeboot-currituck)
     link_address='0x1000000'
     ;;