spi: dt-bindings: stm32: update bindings with SPI Rx DMA-MDMA chaining

Add MDMA channel, and new sram property which are mandatory to enable
SPI Rx DMA-MDMA chaining.

Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Link: https://patch.msgid.link/20250616-spi-upstream-v1-3-7e8593f3f75d@foss.st.com


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