remoteproc: Merge rproc_ops and rproc_fw_ops

There are currently a few different schemes used for overriding fw_ops
or parts of fw_ops. Merge fw_ops into rproc_ops and expose the default
ELF-loader symbols so that they can be assigned by the drivers.

To keep backwards compatibility with the "default" case, a driver not
specifying the "load" operation is assumed to want the full ELF-loader
suit of functions.

Reviewed-By: Loic Pallardy <loic.pallardy@st.com>
Tested-By: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>