ARM: dts: stm32: Drop stm32mp15xc.dtsi from Avenger96

The Avenger96 is populated with STM32MP157A DHCOR SoM, drop the
stm32mp15xc.dtsi which should only be included in DTs of devices
which are populated with STM32MP15xC/F SoC as the stm32mp15xc.dtsi
enables CRYP block not present in the STM32MP15xA/D SoC .

Fixes: 7e76f82a ("ARM: dts: stm32: Split Avenger96 into DHCOR SoM and Avenger96 board")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>