remoteproc: report error if resource table doesn't exist

Currently, if the resource table is completely missing in the
firmware, powering up the remoteproc fails silently. Add a message
indicating that the resource table is missing in the firmware.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>