remoteproc: Support custom firmware handlers

Firmware handling is made customizable.
This is done by creating a separate ops structure for the
firmware functions that depends on a particular firmware
format (such as ELF). The ELF functions are default used
unless the HW driver explicitly injects another firmware
handler by updating rproc->fw_ops.
The function rproc_da_to_va() is exported, as custom
firmware handlers may need to use this function.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
[ohad: namespace fixes, whitespace fixes, style fixes]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>