dts: bindings: spi: Add a new stm32h7 spi compatible

Add a stm32h7 spi compatible.
This compatible intends to match all SPI hardware blocks that
can be compatible with the one available in stm32h7 devices,
for instance, but not limited to stm32u5 and stm32mp1.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>