Documentation: dt: add bindings for TI Wakeup M3 processor

Add the device tree bindings document for the TI Wakeup M3 remote
processor devices on AM33xx and AM43xx SoCs. These devices are used
to offload low-level power management functionality, and are handled
by the wkup_m3 remoteproc driver.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>