soc: qcom: mdt_loader: Split out split-file-loader

Spotted in a SM8450 device, the hash metadata segment is split out in a
separate .bNN file which means that the logic for loading split out
segmenents needs to be duplicated in qcom_mdt_read_metadata().

Split out the existing logic to a helper function that can be used in
both code paths.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>