rtl_nic: add firmware rtl8168fp-3

This adds firmware rtl8168fp-3 for Realtek's RTL8168fp/RTL8117.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Chunhao Lin <hau@realtek.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>