serial: pl011: add mark/space parity support

PL011 UART has hardware mark/space parity ability, this trivial patch adds support for it.
Tested on Raspberry Pi v1, v2 (BCM2835 and BCM2836)

Signed-off-by: Ed Spiridonov <edo.rus@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>