ramips: reduce Archer AX23 / MR70X SPI-frequency

ramips: reduce Archer AX23 / MR70X SPI-frequency

It was brought to attention the Archer AX23 v1 fails to read jffs2 data
from time to time. While this is not reproducible on my unit, it is on
others.

Reducing the SPI frequency does the trick. While it worked with at lest
40 MHz, opt for the cautious side and choose a save frequency of 25 MHz.

Apply the same treatment to the Mercusys MR70X which uses a similar
design just in case.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 2c530fcb972c112e7a2b10f9c21ac6d276624b5e)