remoteproc: debugfs: Add ability to boot remote processor using debugfs

This functionality is especially useful during the testing phase.  When
used in conjunction with Mailbox's Test Framework we can trivially conduct
end-to-end testing i.e. boot co-processor, send and receive messages to
the co-processor, then shut it down again (repeat as required).

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>