dt-bindings: Add device-tree bindings for slimbus slave driver

* BTFM Slimbus Slave Driver

BTFM Slimbus Slave driver configure and initialize slimbus slave device.

Bluetooth SCO and FM Audio data is transferred over slimbus interface.

Required properties:

	- compatible: Should be set to one of the following:

		btfmslim_slave

	- qcom,btfm-slim-ifd: BTFM slimbus slave device entry name

Optional properties:

	- qcom,btfm-slim-ifd-elemental-addr: BTFM slimbus slave device

		enumeration address

Example:

	btfmslim_codec: qca6390 {

		compatible = "qcom,btfmslim_slave";

		elemental-addr = [00 01 20 02 17 02];

		qcom,btfm-slim-ifd = "btfmslim_slave_ifd";

		qcom,btfm-slim-ifd-elemental-addr = [00 00 20 02 17 02];

	};