omap: remoteproc: add a notification for loading error

When calling rproc_get for the first time, the loading of
the remoteproc image will be requested using a non-blocking
request_firmware_no_wait, and the caller can continue before
the actual loading is complete.

The loader later can return an error due to a non-existing
or wrong image and there should be a way to notify about this
to users having a rproc handle.

This functionality is added and is leveraged by rpmsg to
release some resources it had already acquired since requesting
a firmware load.

Change-Id: I1d3523efbcfd613bca74d363084791ceaaaa9989
Signed-off-by: Miguel Vadillo <vadillo@ti.com>

Conflicts:

	arch/arm/plat-omap/omap_rpmsg.c
	include/linux/remoteproc.h