lib: base64: doc: Promote to Utilities doc section

Have base64 helpers show up in the Modules>Utilities section of the doc.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

include/zephyr/sys/base64.h

@@ -31,6 +31,15 @@
 extern "C" {
 #endif
 
+/**
+ * @file
+ *
+ * @defgroup base64 Base64
+ * @brief Base64 encoding/decoding functions
+ * @ingroup utilities
+ * @{
+ */
+
 /**
  * @brief          Encode a buffer into base64 format
  *
@@ -72,6 +81,10 @@ int base64_encode(uint8_t *dst, size_t dlen, size_t *olen, const uint8_t *src,
 int base64_decode(uint8_t *dst, size_t dlen, size_t *olen, const uint8_t *src,
 		  size_t slen);
 
+/**
+ * @}
+ */
+
 #ifdef __cplusplus
 }
 #endif