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>
diff --git a/lib/efi_selftest/efi_selftest_miniapp_exception.c b/lib/efi_selftest/efi_selftest_miniapp_exception.c
index 9be0e7e..79f9a67 100644
--- a/lib/efi_selftest/efi_selftest_miniapp_exception.c
+++ b/lib/efi_selftest/efi_selftest_miniapp_exception.c
@@ -23,7 +23,7 @@
 	struct efi_simple_text_output_protocol *con_out = systable->con_out;
 
 	con_out->output_string(con_out,
-			       L"EFI application triggers exception.\n");
+			       u"EFI application triggers exception.\n");
 
 #if defined(CONFIG_ARM)
 	/*
@@ -38,6 +38,6 @@
 #elif defined(CONFIG_X86)
 	asm volatile (".word 0xffff\n");
 #endif
-	con_out->output_string(con_out, L"Exception not triggered.\n");
+	con_out->output_string(con_out, u"Exception not triggered.\n");
 	return EFI_ABORTED;
 }