TI QSPI controller.

Required properties:
- compatible : should be "ti,dra7xxx-qspi" or "ti,am4372-qspi".
- reg: Should contain QSPI registers location and length.
- reg-names: Should contain the resource reg names.
	- qspi_base: Qspi configuration register Address space
	- qspi_mmap: Memory mapped Address space
	- (optional) qspi_ctrlmod: Control module Address space
- interrupts: should contain the qspi interrupt number.
- #address-cells, #size-cells : Must be present if the device has sub-nodes
- ti,hwmods: Name of the hwmod associated to the QSPI

Recommended properties:
- spi-max-frequency: Definition as per
                     Documentation/devicetree/bindings/spi/spi-bus.txt

Optional properties:
- syscon-chipselects: Handle to system control region contains QSPI
		      chipselect register and offset of that register.

NOTE: TI QSPI controller requires different pinmux and IODelay
paramaters for Mode-0 and Mode-3 operations, which needs to be set up by
the bootloader (U-Boot). Default configuration only supports Mode-0
operation. Hence, "spi-cpol" and "spi-cpha" DT properties cannot be
specified in the slave nodes of TI QSPI controller without appropriate
modification to bootloader.

Example:

For am4372:
qspi: qspi@4b300000 {
	compatible = "ti,am4372-qspi";
	reg = <0x47900000 0x100>, <0x30000000 0x4000000>;
	reg-names = "qspi_base", "qspi_mmap";
	#address-cells = <1>;
	#size-cells = <0>;
	spi-max-frequency = <25000000>;
	ti,hwmods = "qspi";
};

For dra7xx:
qspi: qspi@4b300000 {
	compatible = "ti,dra7xxx-qspi";
	reg = <0x4b300000 0x100>,
	      <0x5c000000 0x4000000>,
	reg-names = "qspi_base", "qspi_mmap";
	syscon-chipselects = <&scm_conf 0x558>;
	#address-cells = <1>;
	#size-cells = <0>;
	spi-max-frequency = <48000000>;
	ti,hwmods = "qspi";
};
