microblaze: move generic-y of exported headers to uapi/asm/Kbuild

microblaze: move generic-y of exported headers to uapi/asm/Kbuild

Since commit fcc8487d477a ("uapi: export all headers under uapi
directories"), all (and only) headers under uapi directories are
exported, but asm-generic wrappers are still exceptions.

To complete de-coupling the uapi from kernel headers, move generic-y
of exported headers to uapi/asm/Kbuild.

With this change, "make headers_install" will just need to parse
uapi/asm/Kbuild to build up exported headers.

For microblaze, "generic-y += siginfo.h" is doubled in asm/Kbuild and
uapi/asm/Kbuild.  So, the one in the former can be simply removed.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>