ath79: dts: fix ja76pf2 spi frequency

The frequency was filled acording the information from datasheet for
particular chip (Winbond 25Q128BVFG). Unfortunately this led to
coruption and introduced bad blocks on the chip. Reducing the frequency
to commonly used in ath79, made the board more stable and no new bad
blocks were spoted.

Fixes: b3a0c97b ("ath79: add support for jjPlus JA76PF2")
Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>