spl.h: make <spl.h> self-contained

The static inline function spl_phase needs <asm/global_data.h>.

Some functions take pointers to struct blk_desc or image_header.
Add forward declarations.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>