dt-bindings: hwlock: qcom-hwspinlock: add syscon to MSM8974

The TCSR_MUTEX region contains two set of registers: mutex and halt.
Add syscon, so the TCSR mutex device (hwspinlock) can use MMIO based
method and in the same time share regmap with other devices for the halt
regs.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Tested-by: Luca Weiss <[e-mail]> # fairphone-fp2
Signed-off-by: Bjorn Andersson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/devicetree/bindings/hwlock/qcom-hwspinlock.yaml

···
               - qcom,apq8084-tcsr-mutex
               - qcom,ipq6018-tcsr-mutex
               - qcom,msm8226-tcsr-mutex
-              - qcom,msm8974-tcsr-mutex
               - qcom,msm8994-tcsr-mutex
           - const: qcom,tcsr-mutex
+      - items:
+          - enum:
+              - qcom,msm8974-tcsr-mutex
+          - const: qcom,tcsr-mutex
+          - const: syscon
 
   reg:
     maxItems: 1