arm64: dts: rockchip: Increase maximum frequency of SPI flash for ROCK Pi 4A/B/C

The ROCK Pi 4A/B/C boards come with a 32 Mbit SPI NOR flash chip (XTX
Technology Limited XT25F32) with a maximum clock frequency of 108 MHz.
Use this value for the device node's spi-max-frequency property.

This patch has been tested on ROCK Pi 4A.

Signed-off-by: Stefan Nagy <stefan.nagy@ixypsilon.net>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>
Link: https://lore.kernel.org/r/20231217113208.64056-1-stefan.nagy@ixypsilon.net
Signed-off-by: Heiko Stuebner <heiko@sntech.de>