ramips: increase SPI frequency for ASUS RT-AC51U/RT-AC54U

ramips: increase SPI frequency for ASUS RT-AC51U/RT-AC54U

This increases the SPI frequency for both ASUS RT-AC51U and RT-AC54U.

Speed comparison tests have been performed on RT-AC54U:

- 10Mhz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    4m 37.78s
user    0m 0.02s
sys     2m 43.92s

- 50Mhz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    1m 28.34s
user    0m 0.03s
sys     0m 46.96s

- 50Mhz fast read
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    1m 11.94s
user    0m 0.01s
sys     0m 46.94s

- 80Mhz
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    1m 12.31s
user    0m 0.04s
sys     0m 46.96s

- 80Mhz fast read
root@OpenWrt:~# time cat /dev/mtd* > /dev/null
real    1m 12.15s
user    0m 0.02s
sys     0m 46.97s

Based on that, we took 50 MHz with fast-read, as higher frequencies
didn't yield further improvements.

For the RT-AC51U, only the final configuration was tested.

Tested-by: Zhijun You <hujy652@gmail.com> [RT-AC54U]
Tested-by: Davide Fioravanti <pantanastyle@gmail.com> [RT-AC51U]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>