ARM: dts: qcom: msm8974: add dedicated IMEM compatible

syscon compatible must be preceded with a specific compatible, to
accurately describe the device.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220607171848.535128-5-krzysztof.kozlowski@linaro.org