gowin: Add a DS location recognition
gowin: Add a DS location recognition

For differential signals it is necessary to set the position of two pins
at once: P and N.
This commit adds that capability and also adds another style of location
setting --- with the pin letter in square brackets used in vendor tools.

Signed-off-by: YRabbit <6d0ebbbdce32474db8141d23d2c01bd9628d6e5f@yrabbit.cyou>

YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr,YosysHQ/nextpnr