Revert "ramips: increase spi-max-frequency for ipTIME mt7620 devices"

Revert "ramips: increase spi-max-frequency for ipTIME mt7620 devices"

This reverts commit 13a185bf8acb67da4a68873e560876c0e60b1a87.

There was a report that one A1004ns device fails to detect its flash
chip correctly:

[    1.470297] spi-nor spi0.0: unrecognized JEDEC id bytes: e0 10 0c 40 10 08
[    1.484110] spi-nor: probe of spi0.0 failed with error -2

It also uses a different flash chip model:
* in my hand: Winbond W25Q128FVSIG (SOIC-8)
* reported: Macronix MX25L12845EMI-10G (SOP-16)

Reducing spi-max-frequency solved the detection failure. Hence revert.

Reported-by: Koasing <koasing@gmail.com>
Tested-by: Koasing <koasing@gmail.com>
Signed-off-by: Sungbo Eo <mans0n@gorani.run>