NMSIS: correcting comment about ECLIC_NEGATIVE_EDGE_TRIGGER

[soburi]

negative edge trigger flags is described as trig[0] == 1 and trig[1] == 1 In ISA Specs document.

https://doc.nucleisys.com/nuclei_spec/isa/eclic.html#eclic-interrupt-source-level-or-edge-triggered-attribute

[fanghuaqi]

LGTM