ramips: increase SPI frequency for MT7620 Archer

ramips: increase SPI frequency for MT7620 Archer

Increase the SPI frequency for the MT7620 based TP-Link Archer
series to 30MHz.

TP-Link uses different SPI flash chips for the same board
revision, so be conservative to not break boards with a
different chip. 30MHz should be well supported by all chips.

Tested on Archer C2 v1 (GD25Q64B) and Archer C20i (W25Q64FV).

Archer C20i (before)
====================
root@OpenWrt:~# time dd if=/dev/mtd1 of=/tmp/test.bin bs=64k
122+0 records in
122+0 records out
real	0m 15.30s
user	0m 0.00s
sys	0m 15.29s

Archer C20i (after)
===================
root@OpenWrt:~# time dd if=/dev/mtd1 of=/tmp/test.bin bs=64k
122+0 records in
122+0 records out
real	0m 5.99s
user	0m 0.00s
sys	0m 5.98s

Signed-off-by: David Bauer <mail@david-bauer.net>
Acked-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>