linux-firmware: ixp4xx: Add the IXP4xx firmware

linux-firmware: ixp4xx: Add the IXP4xx firmware

This firmware has been used since the early 2000s by OpenWrt
and different PDA/SBC projects such as Linux handhelds for
supporting the Network Processing Engines inside the Intel
XScale IXP4xx SoC.

It is better to distribute it inside linux-firmware.

An included program with dubious licensing was removed, we
only redistribute the pre-generated binaries that had clear
licensing.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>