remoteproc: Use size_t type for len in da_to_va

remoteproc: Use size_t type for len in da_to_va

With upcoming changes in elf loader for elf64 support, section size will
be a u64. When used with da_to_va, this will potentially lead to
overflow if using the current "int" type for len argument. Change
da_to_va prototype to use a size_t for len and fix all users of this
function.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Clement Leger <cleger@kalray.eu>
Link: https://lore.kernel.org/r/20200302093902.27849-2-cleger@kalray.eu
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>