mediatek: filogic: increase flash speed on ASUS TUF AX6000

mediatek: filogic: increase flash speed on ASUS TUF 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/mtd1 of=/dev/null bs=10M count=1 status=progress
10485760 bytes (10 MB, 10 MiB) copied, 1.68404 s, 6.2 MB/s

After:
root@OpenWrt:~# dd if=/dev/mtd1 of=/dev/null bs=10M count=1 status=progress
10485760 bytes (10 MB, 10 MiB) copied, 0.819222 s, 12.8 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 <[email protected]>
Signed-off-by: Aleksander Jan Bajkowski <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20853
Signed-off-by: Hauke Mehrtens <[email protected]>