serial: nulldev: Implement "pending" function to fix tstc return "true"

In U-boot, serial_tstc was use to determine is there have a character in
serial console that pending for read. If there is no "pending" function
implemented in serial driver, the serial-uclass will return "true(1)"
to indicate there have a character pending to read.

Thus, read a character from nulldev serial will result in continuous
getting -EAGAIN return which might lead system to hang.

This commit is to fix a bug in nulldev serial which implement "pending"
function in nulldev serial to always indicate there is no character in
console that pending for read.

Signed-off-by: Wilson Lee <wilson.lee@ni.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Keng Soon Cheah <keng.soon.cheah@ni.com>
Cc: Chen Yee Chew <chen.yee.chew@ni.com>
Cc: Bin Meng <bmeng.cn@gmail.com>

drivers/serial/serial_nulldev.c

@@ -18,6 +18,11 @@ static int nulldev_serial_getc(struct udevice *dev)
 	return -EAGAIN;
 }
 
+static int nulldev_serial_pending(struct udevice *dev, bool input)
+{
+	return 0;
+}
+
 static int nulldev_serial_input(struct udevice *dev)
 {
 	return 0;
@@ -36,6 +41,7 @@ static const struct udevice_id nulldev_serial_ids[] = {
 
 const struct dm_serial_ops nulldev_serial_ops = {
 	.putc = nulldev_serial_putc,
+	.pending = nulldev_serial_pending,
 	.getc = nulldev_serial_getc,
 	.setbrg = nulldev_serial_setbrg,
 };