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POWER8 is a specific implementation of ppc64, which is by now outdated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>

Documentation/releases/coreboot-4.9-relnotes.md

@@ -23,6 +23,8 @@ General changes
 * Add bootblock compression capability: on systems that copy the bootblock
   from very slow flash to ERAM, allow adding a stub that decompresses the
   bootblock into ERAM to minimize the amount of flash reads
+* Rename the POWER8 architecture port to PPC64 to reflect that it isn't limited
+  to POWER8
 
 Toolchain
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