arm64: dts: allwinner: A64: Limit MMC2 bus frequency to 150 MHz

In contrast to the H6 (and later) manuals, the A64 datasheet does not
specify any limitations in the maximum possible frequency for eMMC
controllers.
However experimentation has found that a 150 MHz limit similar to other
SoCs and also the MMC0 and MMC1 controllers on the A64 seems to exist
for the MMC2 controller.

Limit the frequency for the MMC2 controller to 150 MHz in the SoC .dtsi.
The Pinebook seems to be the an odd exception, since it apparently seems
to work with 200 MHz as well, so overwrite this in its board .dts file.

Tested on a Pine64-LTS: 200 MHz HS-200 fails, 150 MHz HS-200 works.

Fixes: 22be992f ("arm64: allwinner: a64: Increase the MMC max frequency")
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210113152630.28810-7-andre.przywara@arm.com