clocksource/drivers/ostm: Document renesas-ostm timer DT bindings

* Renesas OS Timer (OSTM)

The OSTM is a multi-channel 32-bit timer/counter with fixed clock

source that can operate in either interval count down timer or free-running

compare match mode.

Channels are independent from each other.

Required Properties:

  - compatible: must be one or more of the following:

    - "renesas,r7s72100-ostm" for the r7s72100 OSTM

    - "renesas,ostm" for any OSTM

		This is a fallback for the above renesas,*-ostm entries

  - reg: base address and length of the register block for a timer channel.

  - interrupts: interrupt specifier for the timer channel.

  - clocks: clock specifier for the timer channel.

Example: R7S72100 (RZ/A1H) OSTM node

	ostm0: timer@fcfec000 {

		compatible = "renesas,r7s72100-ostm", "renesas,ostm";

		reg = <0xfcfec000 0x30>;

		interrupts = <GIC_SPI 102 IRQ_TYPE_EDGE_RISING>;

		clocks = <&mstp5_clks R7S72100_CLK_OSTM0>;

		power-domains = <&cpg_clocks>;

	};