Add delay to let clock stabilize when out of deep sleep

Tests have shown that there is hich-up on MSI clock during the setup phase.
If this stabilization phase happens when application has restarted again
this can have side effects, like grambled UART characters typically.

So we're adding a delay before hading-over back to application.

With this modification, on NCULEO_L476RG, the wake-up time is increased
from 2ms to 2,5ms.
If possible this should be improved in the future to save 500 microseconds
of wak-up time.  See TODO

targets/TARGET_STM/sleep.c

@@ -35,6 +35,18 @@
 extern void HAL_SuspendTick(void);
 extern void HAL_ResumeTick(void);
 
+/*  Wait loop - assuming tick is 1 us */
+static void wait_loop(uint32_t timeout)
+{
+    uint32_t t1, t2, elapsed = 0;
+    t1 = us_ticker_read();
+    do {
+        t2 = us_ticker_read();
+        elapsed = (t2 > t1) ? (t2 - t1) : ((uint64_t)t2 + 0xFFFFFFFF - t1 + 1);
+    } while (elapsed < timeout);
+    return;
+}
+
 // On L4 platforms we've seen unstable PLL CLK configuraiton
 // when DEEP SLEEP exits just few µs after being entered
 // So we need to force MSI usage before setting clocks again
@@ -174,6 +186,12 @@ void hal_deepsleep(void)
     // After wake-up from STOP reconfigure the PLL
     SetSysClock();
 
+    /*  Wait for clock to be stabilized.
+     *  TO DO: a better way of doing this, would be to rely on
+     *  HW Flag. At least this ensures proper operation out of
+     *  deep sleep */
+    wait_loop(500);
+
     TIM_HandleTypeDef TimMasterHandle;
     TimMasterHandle.Instance = TIM_MST;
     __HAL_TIM_SET_COUNTER(&TimMasterHandle, EnterTimeUS);