driver: uart: pl011: fix interrupt driven API

API function:
    - `pl011_irq_tx_enable` is expected to enable and trigger TX interrupt.
    Due to HW limiation, PL011 won't trigger TX interrupt if some data
    wasn't filled to TX FIFO at the beginning. So that `isr_cb` must be
    called at first time to enable TX irq.

    - `pl011_irq_tx_ready` will return true when FIFO can accept more
    data. Here we don't need wait TX FIFO to be empty.

    - `pl011_irq_tx_complete` will return true when all data have been
    sent from the shift register.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>