remoteproc: elf-loader: Add 64 bit elf loading support

The current rproc-elf-loader supports loading of only 32 bit elf files.
Introduce support for loading of 64 bit elf files in rproc-elf-loader.

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