dt-bindings: Add spcom documentation

Qualcomm Technologies, Inc. Secure Proccessor Communication (spcom)

Required properties:

-compatible : should be "qcom,spcom"

-qcom,spcom-ch-names: predefined channels name string

-qcom,spcom-sp2soc-rmb-reg-addr: Secure Processor to SOC shared

                                 register physical address

-qcom,spcom-sp2soc-rmb-initdone-bit: bit indicating Secure

                                     Processor init-done

-qcom,spcom-sp2soc-rmb-pbldone-bit: bit indicating Secure

                                    Processor bootloader-done

-qcom,spcom-soc2sp-rmb-reg-addr: SOC to Secure Processor shared

                                 register physical address

-qcom,spcom-soc2sp-rmb-sp-ssr-bit: bit indicating Secure

                                   Processor subsystem reset

Example:

    qcom,spcom {

            compatible = "qcom,spcom";

            qcom,spcom-ch-names = "sp_kernel" , "sp_ssr";

            qcom,spcom-sp2soc-rmb-reg-addr = <0x01881020>;

            qcom,spcom-sp2soc-rmb-initdone-bit = <24>;

            qcom,spcom-sp2soc-rmb-pbldone-bit = <25>;

            qcom,spcom-soc2sp-rmb-reg-addr = <0x01881030>;

            qcom,spcom-soc2sp-rmb-sp-ssr-bit = <0>;

    };