spiram: expose function to initialize SPI RAM cache

Some frameworks based on ESP-IDF need to be able to decide whether to
initialize SPI RAM after the application has started. This change splits
out part of esp_spiram_init which manipulate cache MMU into a separate
function. Applications can disable cache, call esp_spiram_init_cache,
re-enable cache, and then call esp_spiram_init.
Disabling and re-enabling the cache can be achieved using functions
provided in esp_spi_flash.h.

components/esp32/cpu_start.c

@@ -150,6 +150,7 @@ void IRAM_ATTR call_start_cpu0()
     }
 
 #if CONFIG_SPIRAM_BOOT_INIT
+    esp_spiram_init_cache();
     if (esp_spiram_init() != ESP_OK) {
         ESP_EARLY_LOGE(TAG, "Failed to init external RAM!");
         abort();