MIPS: Loongson: Improve LEFI firmware interface

MIPS: Loongson: Improve LEFI firmware interface

Machtypes of Loongson-3 machines become more and more, but there are
only small differences among different machtypes. Keeping a large table
of machtypes is very ugly and hard to extend. We found that the major
machtype differences are UARTs information (number of UARTs, UART IRQs,
UART clocks, etc.), platform devices (EC, temperature sensors, fan
controllers, etc.) and some workarounds (because of some CPU bugs or
mainboard bugs).

In this patch we improve the UEFI-like (LEFI) interface to make all
Loongson-3 machines use a same machtype "generic-loongson-machine".

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Cc: John Crispin <john@phrozen.org>
Cc: Steven J. Hill <Steven.Hill@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Fuxin Zhang <zhangfx@lemote.com>
Cc: Zhangjin Wu <wuzhangjin@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/8324/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>