remoteproc: Add missing kernel-doc comment for auto-boot

The commit ddf71187 ("remoteproc: Introduce auto-boot flag")
introduced the auto-boot flag but missed adding the corresponding
kernel-doc comment. Add the same.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>