net: ti: icssg: Read firmware name from device-tree

net: ti: icssg: Read firmware name from device-tree

Update the ICSSG PRU Ethernet driver to read PRU/RTU/TXPRU firmware names
from the Device Tree using the "firmware-name" property, instead of relying
on the hard-coded firmware names. The firmware names are parsed during
prueth_probe() and stored in the prueth->firmwares for each slice. The
driver now uses these dynamically loaded names when starting the PRU cores.

This change improves flexibility and allows firmware selection to be
controlled via the device tree, making the driver more adaptable to
different platforms and firmware configurations.

Signed-off-by: MD Danish Anwar <danishanwar@ti.com>