arch/power8: Rename to ppc64

POWER8 is a specific implementation of ppc64, which is by now outdated
(POWER9 has been on the market for a while). Rename arch/power8/ to
potentially cover a wider range of hardware.

TEST=Toolchains built before/after this commit can build coreboot for
     emulation/qemu-power8 from before/after this commit.

Change-Id: I2d6f08b12a9ffc8a652ddcd6f24ad85ecb33ca52
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/c/29943
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Timothy Pearson <tpearson@raptorengineering.com>
diff --git a/src/cpu/qemu-power8/Kconfig b/src/cpu/qemu-power8/Kconfig
index d273cc3..c1f8309 100644
--- a/src/cpu/qemu-power8/Kconfig
+++ b/src/cpu/qemu-power8/Kconfig
@@ -15,7 +15,7 @@
 
 config CPU_QEMU_POWER8
 	bool
-	select ARCH_BOOTBLOCK_POWER8
-	select ARCH_VERSTAGE_POWER8
-	select ARCH_ROMSTAGE_POWER8
-	select ARCH_RAMSTAGE_POWER8
+	select ARCH_BOOTBLOCK_PPC64
+	select ARCH_VERSTAGE_PPC64
+	select ARCH_ROMSTAGE_PPC64
+	select ARCH_RAMSTAGE_PPC64