driver: Timer / Counter initialization incorrect for timer2 with BLE on nRF52840

[carlst]

Describe the bug
An initialization value is incorrect if timer2 is enabled (and used in counter mode) with BLE (where timer0 and timer1 are disabled) on nRF52840.

Please also mention any information which could help others to understand
the problem you're facing:

• drivers/counter/counter_nrfx_timer.c uses a Nordic HAL value to initialize .timer.
  This is out-of-sync with devicetree information as timer0 and timer1 are disabled but
  timer2 is enabled because:
  devicetree
  implies that devicetree starts indexing instances from 0 based on enabled devices, and then indexes disabled devices. So, devicetree index for timer2 (0) and HAL index for timer2 (2) differ.

• see: commit 9a27906

To Reproduce
Steps to reproduce the behavior:

1. Use samples/bluetooth/broadcaster
2. Add following to prj.conf

CONFIG_COUNTER=y
CONFIG_NRFX_TIMER2=y

3. Create boards/nrf52840dk_nrf52840.overlay with:

&timer2 {
  status = "okay";
};

4. Build and run on nRF52840DK
5. Check value of reg in init_timer. It is 0x40008000 and should be 0x4000a000

Expected behavior
timer2 should operate correctly as a counter.

Impact
Very low priority (workaround is to revert commit above).

Environment (please complete the following information):

• OS: Windows
• Toolchain: zephyr-sdk-0.16.5
• Commit SHA or Version used: 5c455aa

[cvinayak]

FYI @aescolar

[aescolar]

(@cvinayak thanks for the ping)
There is a fix for this ready coming in this PR #68618
CC @magp-nordic