ppc: Remove CHRP, POWER3 and POWER4 support from arch/ppc

32-bit CHRP machines are now supported only in arch/powerpc, as are
all 64-bit PowerPC processors.  This means that we don't use
Open Firmware on any platform in arch/ppc any more.

This makes PReP support a single-platform option like every other
platform support option in arch/ppc now, thus CONFIG_PPC_MULTIPLATFORM
is gone from arch/ppc.  CONFIG_PPC_PREP is the option that selects
PReP support and is generally what has replaced
CONFIG_PPC_MULTIPLATFORM within arch/ppc.

_machine is all but dead now, being #defined to 0.

Updated Makefiles, comments and Kconfig options generally to reflect
these changes.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/include/asm-ppc/serial.h b/include/asm-ppc/serial.h
index 485a924..b74af54 100644
--- a/include/asm-ppc/serial.h
+++ b/include/asm-ppc/serial.h
@@ -41,15 +41,10 @@
 #else
 
 /*
- * XXX Assume for now it has PC-style ISA serial ports.
- * This is true for PReP and CHRP at least.
+ * XXX Assume it has PC-style ISA serial ports - true for PReP at least.
  */
 #include <asm/pc_serial.h>
 
-#if defined(CONFIG_MAC_SERIAL)
-#define SERIAL_DEV_OFFSET	((_machine == _MACH_prep || _machine == _MACH_chrp) ? 0 : 2)
-#endif
-
 #endif /* !CONFIG_GEMINI and others */
 #endif /* __ASM_SERIAL_H__ */
 #endif /* __KERNEL__ */