bluetooth: controller: add name to TX power choice

Add a name to the Kconfig choice that selects the controller TX power.
This allows board `*.defconfig` files to change the default TX power
as their certifications warrant. For example:

```
choice BT_CTLR_TX_PWR
	default BT_CTLR_TX_PWR_PLUS_4
endchoice
```

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