tty: hvc: introduce the hv_ops.flush operation for hvc drivers

Use .flush to wait for drivers to flush their console outside of
the spinlock, to reduce lock/irq latencies.

Flush the hvc console driver after each write, which can help
messages make it out to the console after a crash.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>