dts: nordic: nrf54h20: Fix exmif node definition

Remove the "snps,designware-spi" compatible from the EXMIF node in
nRF54H20i, as the spi_dw driver cannot be used for this peripheral
without Nordic-specific modifications that are not present upstream.
An attempt to do so (just setting CONFIG_SPI=y will cause that,
as the driver initialization function will be executed then) results
in a bus fault.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>