net: fm: Add firmware name parameter

In order to read the firmware from the filesystem, we need a file name.
Read the firmware name from the device tree, using the firmware-name
property. This property is commonly used in Linux to determine the
correct name to use (and can be seen in several device trees in U-Boot).

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>