powerpc/Makefile: Don't use $(ARCH) unnecessarily

There's no need to use $(ARCH) for references to the arch directory in
the source tree, it is always arch/powerpc.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231206115548.1466874-1-mpe@ellerman.id.au

arch/powerpc/Makefile

@@ -161,7 +161,7 @@ CFLAGS-y += $(CONFIG_TUNE_CPU)
 
 asinstr := $(call as-instr,lis 9$(comma)foo@high,-DHAVE_AS_ATHIGH=1)
 
-KBUILD_CPPFLAGS	+= -I $(srctree)/arch/$(ARCH) $(asinstr)
+KBUILD_CPPFLAGS	+= -I $(srctree)/arch/powerpc $(asinstr)
 KBUILD_AFLAGS	+= $(AFLAGS-y)
 KBUILD_CFLAGS	+= $(call cc-option,-msoft-float)
 KBUILD_CFLAGS	+= $(CFLAGS-y)
@@ -232,7 +232,7 @@ BOOT_TARGETS2 := zImage% dtbImage% treeImage.% cuImage.% simpleImage.% uImage.%
 
 PHONY += $(BOOT_TARGETS1) $(BOOT_TARGETS2)
 
-boot := arch/$(ARCH)/boot
+boot := arch/powerpc/boot
 
 $(BOOT_TARGETS1): vmlinux
 	$(Q)$(MAKE) $(build)=$(boot) $(patsubst %,$(boot)/%,$@)
@@ -336,7 +336,7 @@ PHONY += $(generated_configs)
 
 define archhelp
   echo '* zImage          - Build default images selected by kernel config'
-  echo '  zImage.*        - Compressed kernel image (arch/$(ARCH)/boot/zImage.*)'
+  echo '  zImage.*        - Compressed kernel image (arch/powerpc/boot/zImage.*)'
   echo '  uImage          - U-Boot native image format'
   echo '  cuImage.<dt>    - Backwards compatible U-Boot image for older'
   echo '                    versions which do not support device trees'
@@ -347,12 +347,12 @@ define archhelp
   echo '                    (your) ~/bin/$(INSTALLKERNEL) or'
   echo '                    (distribution) /sbin/$(INSTALLKERNEL) or'
   echo '                    install to $$(INSTALL_PATH) and run lilo'
-  echo '  *_defconfig     - Select default config from arch/$(ARCH)/configs'
+  echo '  *_defconfig     - Select default config from arch/powerpc/configs'
   echo ''
   echo '  Targets with <dt> embed a device tree blob inside the image'
   echo '  These targets support board with firmware that does not'
   echo '  support passing a device tree directly.  Replace <dt> with the'
-  echo '  name of a dts file from the arch/$(ARCH)/boot/dts/ directory'
+  echo '  name of a dts file from the arch/powerpc/boot/dts/ directory'
   echo '  (minus the .dts extension).'
   echo
   $(foreach cfg,$(generated_configs),