mediatek: filogic: increase flash speed on Netcore N60 Pro

mediatek: filogic: increase flash speed on Netcore N60 Pro

This commit increases the SPI bus frequency from 20 to 52 MHz. Reduces boot
time by 2s. Below is a performance comparison.

spi-max-frequency = <20000000>
> time dd if=/dev/mtd4 of=/dev/null bs=10M count=1
> 1+0 records in
> 1+0 records out
> real    0m 1.86s
> user    0m 0.00s
> sys     0m 0.28s

spi-max-frequency = <52000000>
> time dd if=/dev/mtd4 of=/dev/null bs=10M count=1
> 1+0 records in
> 1+0 records out
> real    0m 1.04s
> user    0m 0.00s
> sys     0m 0.27s

Signed-off-by: st7105 <st7105@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21802
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>