src/arch/ppc64/*: pass FDT address to payload

It's available in %r3 in bootblock and needs to be passed to payload in
%r27.  We use one of two hypervisor's special registers as a buffer,
which aren't used for anything by the code.

Change-Id: I0911f4b534c6f8cacfa057a5bad7576fec711637
Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57084
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Reviewed-by: Krystian Hebel <krystian.hebel@3mdeb.com>

src/arch/ppc64/bootblock_crt0.S

@@ -3,6 +3,8 @@
  * Early initialization code for POWER8/POWER9.
  */
 
+#include <cpu/power/spr.h>
+
 #define FIXUP_ENDIAN						   \
 	tdi   0,0,0x48;	  /* Reverse endian of b . + 8		*/ \
 	b     $+44;	  /* Skip trampoline if endian is good	*/ \
@@ -35,6 +37,10 @@ _start:
 	nop
 	FIXUP_ENDIAN
 
+	/* Store FDT address provided by QEMU in %r3 to pass it later to
+	 * payload */
+	mtspr	SPR_HSPRG0, %r3
+
 	/* Set program priority to medium */
 	or	%r2, %r2, %r2