powerpc: Move cpu_setup_6xx.S and temp.c over to arch/powerpc

Also renamed temp.c to tau_6xx.c (for thermal assist unit) and updated
the Kconfig option description and help text for CONFIG_TAU.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index a433b71..2cdc35c 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -491,7 +491,7 @@
 	  If in doubt, say Y here.
 
 config TAU
-	bool "Thermal Management Support"
+	bool "On-chip CPU temperature sensor support"
 	depends on 6xx
 	help
 	  G3 and G4 processors have an on-chip temperature sensor called the
@@ -500,7 +500,7 @@
 	  on-die temperature in /proc/cpuinfo if the cpu supports it.
 
 	  Unfortunately, on some chip revisions, this sensor is very inaccurate
-	  and in some cases, does not work at all, so don't assume the cpu
+	  and in many cases, does not work at all, so don't assume the cpu
 	  temp is actually what /proc/cpuinfo says it is.
 
 config TAU_INT