arc: m68k: nds32: nios2: sh: xtensa: Add empty spl.h header

At present it is not possible to include spl.h in on these architectures
since the asm/spl.h file is not present. We want to be able to use the
spl_phase() function, so add empty headers to make things build.

Signed-off-by: Simon Glass <sjg@chromium.org>
6 files changed