Look what spiflash_update() is doing:
So, running these commands manually and replacing u-boot.bin with u-boot-meson.bin should be what you’re looking for in this environment.
I don’t see any SPI related modules in your provided lsmod log which are loaded.
In the mainline DTS, SPI flash support is wired out-of-the-box.
So, if the driver isn’t compiled as a build-in, at least spi-meson-spifc should be loaded automatically.
With “modprobe spi-nor” /dev/mtd* should then become available and “modprobe mtdblock” should give /dev/mtdblock*.
In the first step, it is therefore important to make sure that the spi-meson-spifc driver is operational.