[dtgen] Add support for several PLICs

@pamaury is already working on this.

Since Aug 15, 2025.

### Description

Currently `dtgen` assumes that there is only one PLIC. In order to support several PLICs, the code needs to updated. Here is a proposal to do so:
- change the type `dt_plic_irq_id_t`: instead of being alias to `top___irq_id_t`, it should be a structure:
```c
typedef struct dt_plic_irq_id {
dt_instance_id_t plic; // Instance of the PLIC.
union {
top___irq_id_t _id;
...
top___irq_id_t _id;
uint32_t value;
} id;
} dt_plic_irq_id_t;
```
- rv_plic DIF functions should gain a DT version that manipulates `dt_plic_irq_id_t` directly. For example, the following common code becomes incorrect:
```c
rv_plic_testutils_irq_range_enable(
&plic, kPlicTarget,
dt_aon_timer_irq_to_plic_id(kAonTimerDt, kDtAonTimerIrqWkupTimerExpired),
dt_aon_timer_irq_to_plic_id(kAonTimerDt, kDtAonTimerIrqWdogTimerBark));
```
since `dt_aon_timer_irq_to_plic_id` returns a `dt_plic_irq_id_t` and not a `top___irq_id_t`. Fortunately, it's just a matter to create a function with the same name and suffix `_dt`. The compiler will catch all errors automatically because a struct is not convertible to a scalar. The fix is trivial and just needs to replicated in all files (that's a nontrivial amount but the fix is trivial)

An important feature of this design is that `dt_plic_irq_id_t` carries the instance of the PLIC. Thefore if a user tries to call `rv_plic_irq_enable()` on a PLIC which does not correspond to the given `dt_plic_irq_id_t`, this can be detected easily by comparing the PLIC instance IDs, and return an error. Therefore there is no risk of PLIC confusion for code ported to the DT.