bus: stm32_firewall: Simplify with scoped for each OF child loop

bus: stm32_firewall: Simplify with scoped for each OF child loop

Use scoped for-each loop when iterating over device nodes to make code a
bit simpler.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Tested-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260105143657.383621-5-krzysztof.kozlowski@oss.qualcomm.com
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>