ARM: dts: qcom: sdx55: add dedicated IMEM and syscon compatibles

Add proper compatibles to the IMEM device node:
1. syscon to allow accessing memory from other devices,
2. dedicated compatible as required for syscon and simple-mfd nodes.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220607171848.535128-6-krzysztof.kozlowski@linaro.org