remoteproc: Introduce auto-boot flag

Introduce an "auto-boot" flag on rprocs to make it possible to flag
remote processors without vdevs to automatically boot once the firmware
is found.

Preserve previous behavior of the wkup_m3 processor being explicitly
booted by a consumer.

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Loic Pallardy <loic.pallardy@st.com>
Cc: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>