stm32/powerctrl: Save and restore EWUP state when configuring standby.

This allows the user to enable wake-up sources using the EWUP bits, on F7
MCUs.

Disabling the wake-up sources while clearing the wake-up flags follows the
reference manual and ST examples.

ports/stm32/powerctrl.c

@@ -712,10 +712,14 @@ void powerctrl_enter_standby_mode(void) {
     RTC->ISR &= ~ISR_BITS;
 
     #if defined(STM32F7)
+    // Save EWUP state
+    uint32_t csr2_ewup = PWR->CSR2 & (PWR_CSR2_EWUP6 | PWR_CSR2_EWUP5 | PWR_CSR2_EWUP4 | PWR_CSR2_EWUP3 | PWR_CSR2_EWUP2 | PWR_CSR2_EWUP1);
     // disable wake-up flags
     PWR->CSR2 &= ~(PWR_CSR2_EWUP6 | PWR_CSR2_EWUP5 | PWR_CSR2_EWUP4 | PWR_CSR2_EWUP3 | PWR_CSR2_EWUP2 | PWR_CSR2_EWUP1);
     // clear global wake-up flag
     PWR->CR2 |= PWR_CR2_CWUPF6 | PWR_CR2_CWUPF5 | PWR_CR2_CWUPF4 | PWR_CR2_CWUPF3 | PWR_CR2_CWUPF2 | PWR_CR2_CWUPF1;
+    // Restore EWUP state
+    PWR->CSR2 |= csr2_ewup;
     #elif defined(STM32H7)
     EXTI_D1->PR1 = 0x3fffff;
     PWR->WKUPCR |= PWR_WAKEUP_FLAG1 | PWR_WAKEUP_FLAG2 | PWR_WAKEUP_FLAG3 | PWR_WAKEUP_FLAG4 | PWR_WAKEUP_FLAG5 | PWR_WAKEUP_FLAG6;