remoteproc: Add elf64 support in elf loader

In order to support elf64, use macros from remoteproc_elf_helpers.h
to access elf headers depending on elf class.
To allow new drivers to support elf64, add rproc_elf_sanity_check
function which make more sense than adding a elf64 named one since
it will support both elf versions.
Driver which need to support both elf32/elf64 should use this new
function for elf sanity check instead of the elf32 one.

Signed-off-by: Clement Leger <cleger@kalray.eu>
Tested-by: Arnaud POULIQUEN <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20200302093902.27849-7-cleger@kalray.eu


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>