Aside: I am a little hesitant to propose these changes.
If the maintainers decide to reject this pull request
I will understand.
Lightsleep on rp2 is gradually getting more complex
and these changes would contribute to that complexity.

Fixes RP2040 issue micropython#17228 and RP2350 issue micropython#17229.

RP2040 issue is when watchdog timer expires while
lightsleep is running the RP2040 will lock up during
power-on startup.
This occurs because the power-on state machine is configured to
not re-initialize the ROSC and lightsleep stopped the ROSC.

RP2350 issue is the watchdog timer does not decrement while
lightsleep is running.
Once lightsleep returns the timer will start decrementing again.
This occurs because lightsleep clears the bit
CLOCKS_SLEEP_EN1_CLK_SYS_WATCHDOG_BITS while sleeping.

I ran the reproduction tests from the two issues.
I also ran tests/port/rp2/rp2_lightsleep.py.

General alternative. Document that lightsleep and watchdog don't
play well together and leave the code as is.

Alternatives for RP2040 issue micropython#17228:
- Raise issue for pico-sdk.
  Get them to change function _watchdog_enable() so
  the ROSC and XOSC do get reset on a watchdog timeout.
  The existing behavior in _watchdog_enable() has been there
  from day one. There might not be a good reason for it.
  Disadvantage: a possible long wait for a new pico-sdk release.
- Modify lightsleep to not stop the ROSC.
  I measure 4.96 milli-amps when lightsleep disables ROSC and
  5.09 milli-amps when it does not. This is a small change.

Alternatives for the RP2350 issue micropython#17229:
- I don't have any :-).

Signed-off-by: Carl Pottle <cpottle9@outlook.com>