tty: amba-pl011: Add un/throttle support

The PL011 driver lacks throttle and unthrottle methods. As a result,
sending more data to the Pi than it can immediately sink while CRTSCTS
is enabled causes a NULL pointer to be followed.

Add a throttle handler that disables the RX interrupts, and an
unthrottle handler that reenables them.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>