remoteproc: Add mechanism for custom dump function assignment

This patch adds a mechanism for assigning each rproc dump segment with
a custom dump function and private data. The dump function is to be
called for each rproc segment during coredump if assigned.

Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
[bjorn: reordred arguments to rproc_coredump_add_custom_segment()]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>