remoteproc: bail out if firmware has different endianess

At this point we don't support remote processors that have
a different endianess than the host.

Look out for these unsupported scenarios, and bail out if
encountered.

Reported-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Mark Grosen <mgrosen@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>