dt-bindings: Amlogic: add documentation for the SoC info register areas

Amlogic Meson8 and Meson8b "analog top" registers:

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The analog top registers contain information about the so-called

"metal revision" (which encodes the "minor version") of the SoC.

Required properties:

- reg: the register range of the analog top registers

- compatible: depending on the SoC this should be one of:

		- "amlogic,meson8-analog-top"

		- "amlogic,meson8b-analog-top"

		along with "syscon"

Example:

	analog_top: analog-top@81a8 {

		compatible = "amlogic,meson8-analog-top", "syscon";

		reg = <0x81a8 0x14>;

	};

Amlogic Meson6/Meson8/Meson8b assist registers:

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The assist registers contain basic information about the SoC,

for example the encoded SoC part number.

Required properties:

- reg: the register range of the assist registers

- compatible: should be "amlogic,meson-mx-assist" along with "syscon"

Example:

	assist: assist@7c00 {

		compatible = "amlogic,meson-mx-assist", "syscon";

		reg = <0x7c00 0x200>;

	};

Amlogic Meson6/Meson8/Meson8b bootrom:

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The bootrom register area can be used to access SoC specific

information, such as the "misc version".

Required properties:

- reg: the register range of the bootrom registers

- compatible: should be "amlogic,meson-mx-bootrom" along with "syscon"

Example:

	bootrom: bootrom@d9040000 {

		compatible = "amlogic,meson-mx-bootrom", "syscon";

		reg = <0xd9040000 0x10000>;

	};