Xilinx: ARM: UART: clear pending irqs before enabling irqs

The Boot ROM has an issue which will cause the driver to
lock up as pending irqs are not being cleared. With them
cleared it prevents that issue.

This patch is needed for the current (3.9-rc3) mainline kernel. I guess
it went unnoticed, because it was only tested with u-boot up until now.
And u-boot maybe handles this.

[s.trumtrar@pengutronix.de: cherry-picked from linux-xlnx.git]
Signed-off-by: NSteffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/xilinx_uartps.c

@@ -578,6 +578,8 @@ static int xuartps_startup(struct uart_port *port)
 	/* Receive Timeout register is enabled with value of 10 */
 	xuartps_writel(10, XUARTPS_RXTOUT_OFFSET);
 
+	/* Clear out any pending interrupts before enabling them */
+	xuartps_writel(xuartps_readl(XUARTPS_ISR_OFFSET), XUARTPS_ISR_OFFSET);
 
 	/* Set the Interrupt Registers with desired interrupts */
 	xuartps_writel(XUARTPS_IXR_TXEMPTY | XUARTPS_IXR_PARITY |