samples: rtxxx-amp: Add service_area section

Add the service_area section to the list of sections for which objcopy
is executed. As this section was missing from the resulting data image,
it prevents targets (right now, the mimxrt685s/hifi4) from booting.

Signed-off-by: Vit Stanicek <vit.stanicek@nxp.com>
(cherry picked from commit 2d388ba6)