doc: device-tree-bindings: add TIS TPMv2.0 SPI module info

Add TIS TPMv2.0 SPI module bindings.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

doc/device-tree-bindings/tpm2/tis-tpm2-spi.txt

+ST33TPHF20 SPI TPMv2.0 bindings
+-------------------------------
+
+Required properties:
+- compatible		: Should be "tis,tpm2-spi"
+- reg			: SPI Chip select
+
+Optional properties:
+- gpio-reset		: Reset GPIO (if not connected to the SoC reset line)
+- spi-max-frequency	: See spi-bus.txt
+
+Example:
+
+	tpm@1 {
+		compatible = "tis,tpm2-spi";
+		reg = <1>;
+		spi-max-frequency = <10000000>;
+	};