restore rp2040 sleep clocks after explicit light sleep

[dhalbert]

• Fixes rp2040 light_sleep did not restore clocks #8561.

On RP2040, light_sleep_until_alarms() would turn off some clocks during the light sleep by clearing some bits in clocks_hw->sleep_en0 and clocks_hw->sleep_en1. Those clocks are turned off when a wfi() or wfe() happens. But light_sleep_until_alarms() failed to restore the original values of those registers after a light sleep. So the next time a wfi() or wfe() was used, even not during an explicit light sleep, those clocks would get turned off. This caused the clocks to go off during time.sleep() (which uses wfi()), for instance.

To fix, I saved and restored the register values.

Tested with a simplified version of the program in #8561 (I just put an LED on A1).

import alarm
import board
import pwmio
import time

piezo = pwmio.PWMOut(board.A1, duty_cycle=0, frequency=440, variable_frequency=False)
piezo.duty_cycle = 65535 // 2  # On 50%
time.sleep(1)
piezo.duty_cycle = 0 # off

t = 5
print("Sleeping for", 5, "seconds...")
time_alarm = alarm.time.TimeAlarm(monotonic_time=time.monotonic() + t)
alarm.light_sleep_until_alarms(time_alarm)
while True:
    print("Beeping...")
    piezo.duty_cycle = 65535 // 2  # On 50%
    time.sleep(0.25)
    piezo.duty_cycle = 0 # off
    time.sleep(0.25)