remoteproc: Drop firmware_loading_complete

firmware_loading_complete is used to synchronize operations
on rproc while asynchronous firmware loading is in progress.
However, rproc_boot() no longer waits on
firmware_loading_complete. Hence drop this completion
variable altogether and handle the race between rproc_del()
and rproc_boot() using new state RPROC_DELETED.

The request_firmware_nowait() will hold the reference to
rproc device by using a get_device()/put_device(), so the
rproc struct will remain valid even when we return from
rproc_del() before the asynchronous call to
rproc_fw_config_virtio() completes.

CC: Loic Pallardy <loic.pallardy@st.com>
CC: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>