soc: qcom: smem: return proper type for cached entry functions

What phdr_to_last_uncached_entry() returns is the address of the
start of the free space following all allocated uncached entries.
It really doesn't refer to an actual (initialized) private entry
structure.   Similarly phdr_to_last_cached_entry() returns the
address of the end of free space, preceding the last allocated cache
entry.  Change both functions' return type to be pointer to void
to reflect this.

Meanwhile, phdr_to_first_cached_entry() really *does* point to a
private entry structure, so change its return type to reflect
this fact.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>