devicetree: add `DT_ENUM_HAS_VALUE`

Add a macro for checking if an enumeration matches a given value.
This enables code to directly check whether a string enumeration is a
specific value, without needing to construct an intermediate variable,
and without checking against a index value which may change.

```
/* Enables this */
if (DT_INST_ENUM_HAS_VALUE(0, power_amplifier_output, rfo_hp)) {}
/* Instead of this */
if (DT_INST_ENUM_IDX(0, power_amplifier_output) == 0) {}
```

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>