devicetree: Add DT_HAS_ALIAS macro

[rruuaanng]

Add 'DT_HAS_ALIAS' macro to verify node alias existence.

for example

#if DT_HAS_ALIAS(led_gpio)
# define LED_GPIO 0x00
#endif

[rruuaanng]

Good suggestion, I'll change.

[bjarki-andreasen]

Looks good :)

[kartben]

@dkalowsk not sure this qualifies as a bug fix for 4.0? :) To put it differently, I think some reviewers (speaking for myself at least) are expecting to be able to spend less time reviewing new features (albeit small-ish in this case) over the next couple weeks to focus on docs, bug fixes etc. and there is a good chance that these last minute additions will not get as many eyeballs on them as they typically would.

[rruuaanng]

If there are no other issues, after merging this PR, I will modify the rst tag in the documentation regarding device tree errors.

[dkalowsk]

@dkalowsk not sure this qualifies as a bug fix for 4.0? :) To put it differently, I think some reviewers (speaking for myself at least) are expecting to be able to spend less time reviewing new features (albeit small-ish in this case) over the next couple weeks to focus on docs, bug fixes etc. and there is a good chance that these last minute additions will not get as many eyeballs on them as they typically would.

@kartben normally I'd agree. This looks like the addition of a shortcut for a common DT macro mess. That said, it wasn't merged by the time RC2 was cut and thus it now needs an issue attached to it for merging, so moving this to 4.1.