spi: stm32: Add SPI_READY mode to spi controller

The spi ready functionality is supported by our peripheral in host and
target modes on STM32MP2x SoCs.
Update our spi controller driver to allow devices to use this
functionality.

Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Link: https://patch.msgid.link/20250616-spi-upstream-v1-1-7e8593f3f75d@foss.st.com


Signed-off-by: Mark Brown <broonie@kernel.org>