[CRIS] Correct definition of subdirs for install_headers.

Hinko Kocevar noted that the CRIS architecture subdirs were no longer
exported correctly when doing install_headers.

Correct this by exporting all directories as header-y.
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>

include/asm-cris/Kbuild

 include include/asm-generic/Kbuild.asm
 
-header-$(CONFIG_ETRAX_ARCH_V10) += arch-v10/
-header-$(CONFIG_ETRAX_ARCH_V32) += arch-v32/
+header-y += arch/
+header-y += arch-v10/
+header-y += arch-v32/
 
 header-y += ethernet.h
 header-y += rtc.h