fpga-region device tree nodes have the firmware-name property that
contains the file name of firmware in the firmware search path (but not
the path) that shall be loaded before the overlay is applied.

Add the of_firmware_load_overlay() function that accepts an overlay and a
firmware search path, finds the responsible firmware_mgr and loads the
firmware.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>