[XTENSA] Concentrate platforms into one platforms directory.

Create arch/xtensa/platforms/ directory to concentrate
all platforms under that subdirectory and moves the ISS platform
to that directory.

Signed-off-by: Chris Zankel <chris@zankel.net>
diff --git a/arch/xtensa/Makefile b/arch/xtensa/Makefile
index 56685a8..4bd1e14 100644
--- a/arch/xtensa/Makefile
+++ b/arch/xtensa/Makefile
@@ -59,7 +59,7 @@
 head-y		:= arch/xtensa/kernel/head.o
 core-y		+= arch/xtensa/kernel/ arch/xtensa/mm/
 ifneq ($(PLATFORM),)
-core-y		+= arch/xtensa/platform-$(PLATFORM)/
+core-y		+= arch/xtensa/platforms/$(PLATFORM)/
 endif
 libs-y		+= arch/xtensa/lib/ $(LIBGCC)