mediatek: filogic: mt7986: fix SPI driving for NAND and increase to 52MHz on ZyXEL...

mediatek: filogic: mt7986: fix SPI driving for NAND and increase to 52MHz on ZyXEL EX5601

8mA driving will cause overshoot issue on SPI NAND. Change it to 4mA.
- Reference:
https://git01.mediatek.com/plugins/gitiles/openwrt/feeds/mtk-openwrt-feeds/+/003744197aa3a587828b4330ab1112ebdb9e840a

On Linux mainline (mt7986.dtsi), spi's source clock is: clocks = <&topckgen CLK_TOP_MPLL_D2>, which is
208MHz. Usable clock division will be:
- 208/4=52MHz
- 208/6~=35MHz
- 208/8=26MHz
and so on

If we specify 50MHz for spi-max-frequency, it will actually run under about 35MHz. Most SPI NAND & NOR
flashes are capable of running with more than 52MHz, include Micorn MT29F4G01ABAFDWB on ZyXEL EX5601.
[Ref: #18752] To reach highest performance on mt7986, use spi-max-frequency = <520000000>. Basically,
this setting should work on all mt7986 PCBs since most mt7986 boards follow reference design. However,
other boards needs further test to guarantee stability.

Signed-off-by: Sky Huang <SkyLake.Huang@mediatek.com>
Tested-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Link: https://github.com/openwrt/openwrt/pull/18813
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>