Documentation: dt: add bindings for TI Wakeup M3 processor

[ Upstream commit ccbbb9fa ]

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>