efi: Use 16-bit unicode strings

At present we use wide characters for unicode but this is not necessary.
Change the code to use the 'u' literal instead. This helps to fix build
warnings for sandbox on rpi.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

lib/efi_loader/efi_string.c

@@ -16,7 +16,7 @@
  * @index:	Index
  *
  * Create a utf-16 string with @name, appending @index.
- * For example, L"Capsule0001"
+ * For example, u"Capsule0001"
  *
  * The caller must ensure that the buffer has enough space for the resulting
  * string including the trailing L'\0'.