ARM: dts: stm32: update SPI6 dmas property on stm32mp157c

Remove unused parameter from SPI6 dmas property on stm32mp157c SoC.

Fixes: dc3f8c86 ("ARM: dts: stm32: add SPI support on stm32mp157c")
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
[olof: Without this patch, SPI6 will fall back to interrupt mode with
lower perfmance]
Signed-off-by: Olof Johansson <olof@lixom.net>