MAINTAINERS: Add mailing list for remote processor subsystems

Add the newly created linux-remoteproc mailing list for the three
subsystems related to remote processor management and communication.

Acked-by: Ohad Ben-Cohen <ohad@wizery.com>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>