lantiq-gphy-firmware: add package

lantiq-gphy-firmware: add package

This has several advantages:
* reduction in the size of the kernel and the complete image. Individual
devices only need two of the four binaries. In combination with the second
commit it reduces kernel size by 64.2 kB and image size by 22.8 kB,
* the option to extend this package with firmware for future SoCs,
* combining the kernel and binary blobs with another licence may not be
fully compatible with the licence used by Linux. The current PHY firmware
is built into the kernel. This comit converts it to a package.

Tested on AVM 5490 and BT Home Hub 5A.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Link: https://github.com/openwrt/openwrt/pull/17669
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>