serial: mvebu-uart: add TX interrupt trigger for pulse interrupts

Pulse interrupts (extended UART only) needs a change of state to trigger
the TX interrupt. In addition to enabling the TX_READY_INT_EN flag,
produce a FIFO state change from 'empty' to 'not full'. For this, write
only one data byte in TX start, making the TX FIFO not empty, and wait
for the TX interrupt to continue the transfer.

Signed-off-by: Allen Yan <yanwei@marvell.com>
Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>