kernel: Add new platform EcoNet MIPS

kernel: Add new platform EcoNet MIPS

EcoNet EN75xx is a big endian MIPS platform used in XPON (fiber),
DSL, and SIM (3g/4g) applications. Complete GPL vender SDKs exist
for this platform, but are based on Linux 2.6.

The bulk of this submission has already been accepted upstream:
https://patchwork.kernel.org/project/linux-mips/list/?series=960479&state=*

This platform uses a bootloader that is derived from old TrendChip
code. This bootloader implements a frustratingly complex Bad Block
Table which is implemented here in en75_bmt.c

This BMT is not upstreamed because it depends on mtk_bmt framework
which likewise is not upstreamed.

This BMT system rewrites block indexes in flash and if the bootloader
considers it to be corrupted, it will attempt to automatically rebuild
on boot. So without implementing the algorithm, you can't safely use
the disk at all.

Signed-off-by: Caleb James DeLisle <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19021
Signed-off-by: Hauke Mehrtens <[email protected]>