doc: lpspi: Document DT bindings for LPSPI clocks

- reg : address and length of the lpspi master registers

- interrupt-parent : core interrupt controller

- interrupts : lpspi interrupt

- clocks : lpspi clock specifier

- clocks : lpspi clock specifier. Its number and order need to correspond to the

	   value in clock-names.

- clock-names : Corresponding to per clock and ipg clock in "clocks"

		respectively. In i.MX7ULP, it only has per clk, so use CLK_DUMMY

		to fill the "ipg" blank.

- spi-slave : spi slave mode support. In slave mode, add this attribute without

	      value. In master mode, remove it.

	reg = <0x40290000 0x10000>;

	interrupt-parent = <&intc>;

	interrupts = <GIC_SPI 28 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&clks IMX7ULP_CLK_LPSPI2>;

	clocks = <&clks IMX7ULP_CLK_LPSPI2>,

		 <&clks IMX7ULP_CLK_DUMMY>;

	clock-names = "per", "ipg";

	spi-slave;

};