Bluetooth: controller: Kconfig TX power dBm

Adds a Kconfig symbol which contains the default controller TX power
directly in dBm. This allows code to directly display/use the configured
power, instead of having to manually iterate over all the
`BT_CTLR_TX_PWR` options.

```
  LOG_INF("BT controller configured for %ddBm TX power",
          CONFIG_BT_CTLR_TX_PWR_DBM);
```

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