drivers: intc: plic: add shell cmd to get irq stats for debugging

Introduced `CONFIG_PLIC_SHELL` to enable the build of shell
debugging command to get the hit count of each interrupt
controller's IRQ line. This is especially useful when working
with dynamically installed ISRs, which will be the case for
`plic_sw`.

Example usage:

```
uart:~$ plic stats get interrupt-controller@c000000
   IRQ        Hits
==================
    10         177

uart:~$ plic stats get interrupt-controller@c000000
   IRQ        Hits
==================
    10         236

uart:~$ plic stats clear interrupt-controller@c000000
Cleared stats of interrupt-controller@c000000.

uart:~$ plic stats get interrupt-controller@c000000
   IRQ        Hits
==================
    10          90

```

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Maxim Adelman <imax@meta.com>