[Pull Request] New Platform: EcoNet EN75xx MIPS

[Caleb James DeLisle]

Hello all,

I've just created a PR on Github to add a new platform, EN75xx MIPS.
https://github.com/openwrt/openwrt/pull/17928

I made my best effort to explain the design decisions in the commit
message itself, so I'll use this message to explain on a higher level
the platform and my objectives for it.

As I noted in the commit message, this is my first contribution so I
would most appreciate any help I can get in moving it down the road
toward being merge-able.

The SmartFiber XP8421-B is a sub-$20 device available from Alibaba and
it has a 900Mhz 34Kc Big Endian MIPS processor, 512M memory, 256M SPI
NAND flash, 2x USB 2.0, 4x Gb Ethernet, Wifi MT7603 + MT7662E, XPON,
and VoIP DSP.

EcoNet used to be a subsidiary of MediaTek which produced telecom
focused products before merging with Airoha, another subsidiary with-whom
EcoNet shared a lot of IP. While they share a number of drivers,
Airoha focused on the ARM(el) architecture while EcoNet focused on
MIPS(eb).

The airoha target on OpenWRT is an ARM target which is supporting
their current flagship AN7581, my goal with introducing the en75
target is to support the EcoNet MIPS devices which will allow
up-cycling of telecom e-waste and the use of bargain-bin devices based
on EOL'd silicon.

There are two sub-targets of the en75 target, those are en751221 and