remoteproc: add find_loaded_rsc_table firmware ops

Add function find_loaded_rsc_table to firmware ops. This function
returns the location of the resource table in shared memory
after loading.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Acked-by: Ido Yariv <ido@wizery.com>
[align function name with existing terminology, update commit log]
[document new function, rebase patch, small cleanups]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>