mailbox: qcom-apcs-ipc: do not grow the of_device_id

Re-organize the compatible devices and add a comment to avoid unneeded
of_device_id growth with every new SoC.  These devices have quite a lot
of similarities and they can use only one compatible fallback for driver
binding.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>