ath79: mikrotik: fix reference clock of Routerboard 911G

ath79: mikrotik: fix reference clock of Routerboard 911G

When support for Routerboard 911G was introduced, Routerboad 912UAG
device tree was used as a base, and the common part. This led to use of
40MHz as the reference clock frequency for both [1], while RB911G uses 25MHz
crystal on the board, causing heavy system clock drift.

Split the definition, and set the reference clock frequency for RB911G
back to 25MHz.

[1] a716ac556497 ("ath79: fix reference clock for RouterBoard 912UAG")

Fixes: bcc44b1212b2 ("ath79: support for MikroTik RouterBOARD 911G-(2,5)HPnD")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17944
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>