riscv: cosmetic: Reword do_reset() printf message.

The Sentence "reset unsupported yet" is not
grammatically correct and should say
"reset not supported yet" instead.

Suggested-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Signed-off-by: Rick Chen <rick@andestech.com>

arch/riscv/lib/reset.c

@@ -10,7 +10,7 @@ int do_reset(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 {
 	printf("resetting ...\n");
 
-	printf("reset unsupported yet\n");
+	printf("reset not supported yet\n");
 	hang();
 
 	return 0;