Device-Tree bindings for MediaTek MT5896 SoC based KeyPad

Required properties:
- compatible	    : Should be
			"mediatek,mt5896-spi" : for MT5896 SoC
- reg 		    : Specifies base physical address and size of the registers;
- mtk, keypad-chanel : keypad PCB channel
- mtk, keypad-lowbd  : keypad low bound adc trigger interrupt
- mtk, keypad-upwbd  : keypad high bound adc trigger interrupt
- mtk, keypad-num    : keypad PCB key supported

Example:

	keypad {
		compatible = "mediatek,mt5896-keypad";
		interrupt-parent = <&mtk_intc2_irq_high>;
		reg = <0 0x1f020a00 0 0x200>;
		interrupts = <0x0 0x10 0x4>;
		mtk,keypad-chanel = <1>;
		mtk,keypad-lowbd = <848>;
		mtk,keypad-upwbd = <1023>;
		mtk,keypad-num = <2>;

		key_1 {
			linux,code = <0x8b>;
			keypad,threshold = <0x339>;
		};
		key_2 {
			linux,code = <0x4F>;
			keypad,threshold = <0x2DB>;
		};
	};
