ARM: dts: qcom: apq8064-asus-nexus7: 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>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220607171848.535128-4-krzysztof.kozlowski@linaro.org