dts: arm: st: stm32h5: fix spi 1-3 clocks

The STM32 SPI driver, `spi_ll_stm32.c`, reads the clock frequency via
`clock_control_get_rate()`. The first `clocks` index is used as subsystem
argument if there is no second index, but this is not the source clock for
SPI 1, 2, and 3.
This causes the prescaler value calculation to be incorrect, resulting in a
frequency potentially above the `spi-max-frequency` value.

Add clock source for SPI instances 1, 2 and 3, that matches the default
clock configuration register reset value, which resolves the issue.

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>