[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 <[e-mail]>

arch/xtensa/Makefile

···
 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)