Adding OpenWrt support for Asus RT-AC57U v2

CPU data:

admin@RT-AC57U_V2:/tmp/home/root# cat /proc/cpuinfo
system type             : Qualcomm Atheros QCA5502 ver 1 rev 0
machine                 : ASUS RT-AC59U
processor               : 0
cpu model               : MIPS 74Kc V5.0
BogoMIPS                : 385.84
wait instruction        : yes
microsecond timers      : yes
tlb_entries             : 32
extra interrupt vector  : yes
hardware watchpoint     : yes, count: 4, address/irw mask: [0x0ffc, 0x0ffc, 0x0ffb, 0x0ffb]
isa                     : mips1 mips2 mips32r1 mips32r2
ASEs implemented        : mips16 dsp dsp2
shadow register sets    : 1
kscratch registers      : 0
package                 : 0
core                    : 0
VCED exceptions         : not available
VCEI exceptions         : not available

admin@RT-AC57U_V2:/tmp/home/root# uname -a
Linux RT-AC57U_V2 4.4.60 #1 Wed Jan 6 15:07:10 CST 2021 mips GNU/Linux

Comparing with the other forum thread (Add support for Asus RT-AC57U_V2) cpu is sligtly different.

"Qualcomm Atheros QCA5502 rev 0" VS "Qualcomm Atheros QCA5502 ver 1 rev 0"

Sligtly different capabilities:
isa : mips1 mips2 mips32r1 mips32r2
ASEs implemented : mips16 dsp dsp2

VS
ASEs implemented : mips16 dsp

I think it is not a big deal, and the two device is identical.

I upgraded to the newest Asus firmware (3.0.0.4.382_52482), what use 4.4.60 kernel.