serial: amba-pl011: Fix serial port discard interrupt when interrupt signal line of serial port is connected to mbigen.

ascend inclusion
category: bugfix
Bugzilla: N/A
CVE: N/A

---------------------------------------

Hisi when designing ascend chip, connect the serial port interrupt
signal lines to mbigen equipment, mbigen write GICD_SETSPI_NSR
register trigger the SPI interrupt. This can result in serial
port drop interrupts.
Signed-off-by: NXu Qiang <xuqiang36@huawei.com>
Reviewed-by: NHanjun Guo <guohanjun@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>