stm32/modmachine: Re-enable PLLSAI[1] after waking from stop mode.

On F7s PLLSAI is used as a 48MHz clock source if the main PLL cannot
provide such a frequency, and on L4s PLLSAI1 is always used as a clock
source for the peripherals.  This commit makes sure these PLLs are
re-enabled upon waking from stop mode so the peripherals work.

See issues #4022 and #4178 (L4 specific).

ports/stm32/modmachine.c

@@ -568,6 +568,22 @@ STATIC mp_obj_t machine_sleep(void) {
     #endif
     }
 
+    #if defined(STM32F7)
+    if (RCC->DCKCFGR2 & RCC_DCKCFGR2_CK48MSEL) {
+        // Enable PLLSAI if it is selected as 48MHz source
+        RCC->CR |= RCC_CR_PLLSAION;
+        while (!(RCC->CR & RCC_CR_PLLSAIRDY)) {
+        }
+    }
+    #endif
+
+    #if defined(STM32L4)
+    // Enable PLLSAI1 for peripherals that use it
+    RCC->CR |= RCC_CR_PLLSAI1ON;
+    while (!(RCC->CR & RCC_CR_PLLSAI1RDY)) {
+    }
+    #endif
+
     #endif
 
     return mp_const_none;