net: cortina: Add support for tfa boot in cortina firmware

Add support for boards supporting TFA boot separately in cortina
firmware. Please note, a weak function is defined to retrieve firmware
address values as CONFIG_CORTINA_FW_ADDR is now defined in defconfig and
can only have one possible value defined. This weak function will help
in overwrting the values to get proper addresses as per boot source.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>