bsps/include/dev/spi: Add missing extern and guard

This adds missing extern "C" declarations and a missing header guard.

bsps/include/dev/spi/xqspi_flash.h

@@ -31,6 +31,10 @@
 #include <dev/flash/flashdev.h>
 #include <dev/spi/xqspipsu.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define XQSPI_FLASH_MAX_REGIONS ((size_t)32)
 
 /*
@@ -65,4 +69,8 @@ typedef struct xqspi_flash_region_table {
   uint32_t xqspi_flash_bit_allocator;
 } xqspi_flash_region_table;
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* XILINX_XQSPI_FLASH_H */