remoteproc: refactor rproc_elf_find_rsc_table()

Refactor rproc_elf_find_rsc_table() and split out the scanning
for the section header named resource table. This is done to
prepare for loading firmware once.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Acked-by: Ido Yariv <ido@wizery.com>
[small function name change to make the code easier to read]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>