remoteproc: add elf file load support

remoteproc: add elf file load support

The current implementation supports only binary file load.
Add helpers to support ELF32 format (sanity check, and load).
Note that since an ELF32 image is built for the remote processor, the
load function uses the device_to_virt ops to translate the addresses.
Implement a basic translation for sandbox_testproc.

Add related tests. Test result:
=> ut dm remoteproc_elf
Test: dm_test_remoteproc_elf: remoteproc.c
Test: dm_test_remoteproc_elf: remoteproc.c (flat tree)
Failures: 0

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>