tty: amba-pl011: allow shared interrupt

The PL011 register space includes all necessary status bits to
determine whether a device instance requires handling in response
to an interrupt. Therefore, multiple instances of the device could
be serviced by a single shared interrupt, which is the case on BCM7211.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>