remoteproc: elf_loader: Introduce rproc_elf_get_boot_addr() api

Introduce rproc_elf_get_boot_addr() that returns the entry point of
the elf file. This api auto detects the 64/32 bit elf file and returns
the boot addr accordingly.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>