xen/arm: Manage pl011 uart TX interrupt correctly

xen/arm: Manage pl011 uart TX interrupt correctly

In pl011.c, when TX interrupt is received
serial_tx_interrupt() is called to push next
characters. If TX buffer is empty, serial_tx_interrupt()
does not disable TX interrupt and hence pl011 UART
irq handler pl011_interrupt() always sees TX interrupt
status set in MIS register and cpu does not come out of
UART irq handler.

With this patch, mask TX interrupt by writing 0 to
IMSC register when TX buffer is empty and unmask by
writing 1 to IMSC register before sending characters.

Signed-off-by: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>
Reviewed-by: Tim Deegan <tim@xen.org>