dt-bindings: arm: stm32: add extra SiP compatible for lxa,stm32mp157c-mc1

The Linux Automation MC-1 is built around an OSD32MP15x SiP with CPU,
RAM, PMIC, Oscillator and EEPROM. Adjust the binding, so the SiP
compatible is contained as well. This allows boot firmware to match
against it to apply fixups if necessary.

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