remoteproc: Add function rproc_get_boot_addr

Prepare for introduction of custom firmware loaders by
moving the function operating on ELF data-structures into
separate functions. Move lookup of the boot_addr in the
ELF binary to the function rproc_get_boot_addr().

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
[rproc_get_boot_addr's kerneldoc: add missing @rproc line]
[rproc_get_boot_addr's kerneldoc: minor style changes]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>