variants: Introduce the builtin-led-gpios node

[soburi]

Introduce the builtin-led-gpios node to make possible to configuring
the builtin-led from dts.
This node uses to determine the value of the LED_BUILTIN macro.

Use the led0 alias to determine the LED_BUILTIN if the builtin-led-gpios
is not defined.

The behavior can override by defining the LED_BUILTIN in pinmap.h.
It uses definitions that are defined in the header preferentially.

---

The PR is contineued from #60.
I will rebase if #60 merged.

[szczys]

This is a nice improvement and the changes LGTM.

This PR should also include an update to the variant documentation on this topic:

gsoc-2022-arduino-core/documentation/variants.md

Lines 57 to 84 in 5c305c6

	### Overlays using previously-defined Arduino headers
	When an Arduino header exists in a board's in-tree DTS file it can easily be
	used to create the necessary overlay file. Assign the relevant mapping using the
	Arduino header label (usually either `&arduino_header` or `&arduino_nano_header`
	and the `gpio_map` number. The second number is used to add GPIO flags and may
	safely be left as zero.
	For example, creating an overlay file for the Nordic nRF52840 Development Kit
	uses [the Arduino header definitions](https://github.com/zephyrproject-rtos/zephyr/blob/6f8ee2cdf7dd4d746de58909204ea0ce156d5bb4/boards/arm/nrf52840dk_nrf52840/nrf52840dk_nrf52840.dts#L74-L101), beginning with the first digital pin:
	```
	/ {
	zephyr,user {
	d0_gpios = <&arduino_header 6 0>; /* Digital */
	d1_gpios = <&arduino_header 7 0>;
	...
	d13_gpios = <&arduino_header 19 0>;
	d14_gpios = <&arduino_header 0 0>; /* Analog */
	d15_gpios = <&arduino_header 1 0>;
	...
	d19_gpios = <&arduino_header 5 0>;
	d20_gpios = <&arduino_header 20 0>; /* SDA */
	d21_gpios = <&arduino_header 21 0>; /* SCL */
	d22_gpios = <&gpio0 13 GPIO_ACTIVE_LOW>; /* LED0 */
	};
	};
	```

[soburi]

@szczys

Thank you for reviewing.

This PR should also include an update to the variant documentation on this topic:

I update the document.

[DhruvaG2000]

This makes the overall code around builtin LEDs much more generic and future proof. Great addition!

Tested-by: Dhruva Gole