dt-bindings: arm: stm32: Convert STM32 SoC bindings to DT schema

This commit converts STM32 SoC bindings to DT schema using jsonschema.
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

Documentation/devicetree/bindings/arm/stm32/stm32.txt

-STMicroelectronics STM32 Platforms Device Tree Bindings
-
-Each device tree must specify which STM32 SoC it uses,
-using one of the following compatible strings:
-
-  st,stm32f429
-  st,stm32f469
-  st,stm32f746
-  st,stm32h743
-  st,stm32mp157

Documentation/devicetree/bindings/arm/stm32/stm32.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/stm32/stm32.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: STMicroelectronics STM32 Platforms Device Tree Bindings
+
+maintainers:
+  - Alexandre Torgue <alexandre.torgue@st.com>
+
+properties:
+  compatible:
+    oneOf:
+      - items:
+          - const: st,stm32f429
+
+      - items:
+          - const: st,stm32f469
+
+      - items:
+          - const: st,stm32f746
+
+      - items:
+          - const: st,stm32h743
+
+      - items:
+          - const: st,stm32mp157
+...