clk: at91: update main clk documentation

		All at91 specific clocks (clocks defined below) must be child

		node of the PMC node.

	"atmel,at91rm9200-clk-main-osc"

	"atmel,at91sam9x5-clk-main-rc-osc"

		at91 main clk sources

	"atmel,at91sam9x5-clk-main"

	"atmel,at91rm9200-clk-main":

		at91 main oscillator

		at91 main clock

	"atmel,at91rm9200-clk-master" or

	"atmel,at91sam9x5-clk-master":

		/* put at91 clocks here */

	};

Required properties for main clock internal RC oscillator:

- interrupt-parent : must reference the PMC node.

- interrupts : shall be set to "<0>".

- clock-frequency : define the internal RC oscillator frequency.

Optional properties:

- clock-accuracy : define the internal RC oscillator accuracy.

For example:

	main_rc_osc: main_rc_osc {

		compatible = "atmel,at91sam9x5-clk-main-rc-osc";

		interrupt-parent = <&pmc>;

		interrupts = <0>;

		clock-frequency = <12000000>;

		clock-accuracy = <50000000>;

	};

Required properties for main clock oscillator:

- interrupt-parent : must reference the PMC node.

- interrupts : shall be set to "<0>".

- #clock-cells : from common clock binding; shall be set to 0.

- clocks : shall encode the main osc source clk sources (see atmel datasheet).

Optional properties:

- atmel,osc-bypass : boolean property. Specified if a clock signal is provided

  on XIN.

  clock signal is directly provided on XIN pin.

For example:

	main_osc: main_osc {

		compatible = "atmel,at91rm9200-clk-main-osc";

		interrupt-parent = <&pmc>;

		interrupts = <0>;

		#clock-cells = <0>;

		clocks = <&main_xtal>;

	};

Required properties for main clock:

- interrupt-parent : must reference the PMC node.

- interrupts : shall be set to "<0>".

- #clock-cells : from common clock binding; shall be set to 0.

- clocks (optional if clock-frequency is provided) : shall be the slow clock

	phandle. This clock is used to calculate the main clock rate if

	"clock-frequency" is not provided.

- clock-frequency : the main oscillator frequency.Prefer the use of

	"clock-frequency" over automatic clock rate calculation.

- clocks : shall encode the main clk sources (see atmel datasheet).

For example:

	main: mainck {

		compatible = "atmel,at91rm9200-clk-main";

		compatible = "atmel,at91sam9x5-clk-main";

		interrupt-parent = <&pmc>;

		interrupts = <0>;

		#clock-cells = <0>;

		clocks = <&ck32k>;

		clock-frequency = <18432000>;

		clocks = <&main_rc_osc &main_osc>;

	};

Required properties for master clock: