dt-bindings: arm: stm32: document lxa,stm32mp157c-mc1 compatible

Document the STM32MP157 based Linux Automation MC-1 device tree
compatible.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>