ARM: dts: stm32: fix compatible for BSEC on STM32MP13

Use the correct compatible for stm32mp13 support.

The BSEC driver for STM32MP15x is not compatible with STM32MP13x. For
example the proprietary's smc STM32_SMC_BSEC is not supported in
STM32MP13x OP-TEE, it is replaced by SM32MP BSEC Pseudo Trusted
Application in OP-TEE to access to the secured IP BSEC on STM32MP13X SoC.

The correct compatible is already used in U-Boot and in upstream is in
progress for OP-TEE device tree.

As the SoC STM32MP13X is not yet official and it is not available
outside STMicroelectronics, it is the good time to break the DTS
compatibility and to correct the error done in the introduction of
STM32MP131.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>