[Buildroot] stm32mp1 kernel won't boot on custom board

[Ra C]

I have a custom stm32mp157a board which runs OK with a modified 5.4 kernel.
I want to migrate to the latest stm supported kernel.
I built a stm32mp157a_dk1 target using the BR2_EXTERNAL feature described
here: https://github.com/bootlin/buildroot-external-st which builds a 5.15
kernel.

Using the same dtb , the 5.15 kernel fails to mount the root device (a
micro sd card on SDMMC1) and gets stuck in a loop looking for a device
matching the PARTUUID in class_find_device().  The only devices listed are
loop and ram.

The probe function in drivers/mmc/host/mmci.c, which gets called in the
working kernel prior to mounting the root device does not get called with
the 5.15 kernel.

I don't understand why this is happening given that the same device tree is
used in both cases and not sure where next to look.
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