mediatek: filogic: increase nand flash speed on Redmi AX6000

mediatek: filogic: increase nand flash speed on Redmi AX6000

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

Before:
root@OpenWrt:~# dd if=/dev/mtd5 of=/dev/null bs=10M count=1 status=progress
10485760 bytes (10 MB, 10 MiB) copied, 2 s, 5.8 MB/

After:
root@OpenWrt:~# dd if=/dev/mtd5 of=/dev/null bs=10M count=1 status=progress
10485760 bytes (10 MB, 10 MiB) copied, 1 s, 9.7 MB/s

Taken from PR #18752 as each device should be tested individually, so I have
created a separate PR for this.

Signed-off-by: Sky Huang <SkyLake.Huang@mediatek.com>
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Link: https://github.com/openwrt/openwrt/pull/19439
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>