nds32: remove redundant kernel-space generic-y

This commit removes redundant generic-y defines in
arch/nds32/include/asm/Kbuild.

[1] It is redundant to define the same generic-y in both
    arch/$(ARCH)/include/asm/Kbuild and
    arch/$(ARCH)/include/uapi/asm/Kbuild.

    Remove the following generic-y:

      bitsperlong.h
      bpf_perf_event.h
      errno.h
      fcntl.h
      ioctl.h
      ioctls.h
      mman.h
      shmbuf.h
      stat.h

[2] It is redundant to define generic-y when arch-specific
    implementation exists in arch/$(ARCH)/include/asm/*.h

    Remove the following generic-y:

      ftrace.h

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