soc: qcom: smem: introduce qcom_smem_partition_header()

Create a new function qcom_smem_partition_header() to encapsulate
validating locating a partition header and validating information
found within it.  This will be built up over a few commits to make
it more obvious how the common function is replacing duplicated code
elsewhere.  Initially it just verifies the header has the right
magic number.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>