[Core wide] custom drivers entry for owntech.

[jalinei]

At the moment owntech custom drivers override the behavior of zephyr drivers.
It would be a great enhancement to change that behavior so owntech drivers change the compatible in device tree.

		usart3: serial@40004800 {
			compatible = "st,stm32-usart", "st,stm32-uart";
			reg = < 0x40004800 0x400 >;
			clocks = <&rcc 0x58 0x40000 >;
			resets = < &rctl 0x712 >;
			interrupts = < 0x27 0x0 >;
			status = "okay";
			pinctrl-0 = < &usart3_tx_pc10 &usart3_rx_pc11 >;
			pinctrl-names = "default";
			current-speed = < 0xa21fe8 >;
		};

For instance here we would like to have usart3 tied to rs485, that is supported by our own real time driver. Hence we expect compatible to be :
compatible = "st,stm32-uart-rt-owntech";

Also we should at some point use the dma instructions so the OS can keep track of the channels that are reserved to the owntech real time drivers.