esp_system: add VDD_SDIO power domain configuration for light sleep

VDD_SDIO power domain can now be configured for light sleep
by the application. It is now possible to keep the power domain
ON during light sleep, keeping the GPIOs connected to it powered.
The power domain will, by default be:
- Kept ON if CONFIG_ESP_SYSTEM_PD_FLASH is not set
- Turned OFF if not set

The application can still force it to be ON by calling
`esp_sleep_pd_config(ESP_PD_DOMAIN_VDDSDIO, ESP_PD_OPTION_ON);`

components/esp_system/include/esp_sleep.h

@@ -50,6 +50,7 @@ typedef enum {
     ESP_PD_DOMAIN_RTC_FAST_MEM,    //!< RTC fast memory
     ESP_PD_DOMAIN_XTAL,            //!< XTAL oscillator
     ESP_PD_DOMAIN_CPU,             //!< CPU core
+    ESP_PD_DOMAIN_VDDSDIO,         //!< VDD_SDIO
     ESP_PD_DOMAIN_MAX              //!< Number of domains
 } esp_sleep_pd_domain_t;