remoteproc: fix missing CONFIG_FW_LOADER configurations

Remoteproc requires user space firmware loading support, so
let's select FW_LOADER explicitly to avoid painful misconfigurations
(which only show up in runtime).

Cc: stable <stable@vger.kernel.org>
Reported-by: Mark Grosen <mgrosen@ti.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>