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/include/cpu/power/spr.h

@@ -9,6 +9,9 @@
 #define SPR_PVR_REV_MASK			(PPC_BITMASK(52, 55) | PPC_BITMASK(60, 63))
 #define SPR_PVR_REV(maj, min)			(PPC_SHIFT((maj), 55) | PPC_SHIFT((min), 63))
 
+#define SPR_HSPRG0				0x130
+#define SPR_HSPRG1				0x131
+
 #define SPR_HRMOR				0x139
 
 #define SPR_HMER				0x150