YANG updates for IETF 118

[italobusi]

Updated flexi-grid-label-hop: fix #33

Updated name for the flexi-grid channel spacing: fix #56

• deprecated the flexi-ch-spc-type and flexi-ch-spc-6p25ghz identities;
• deprecated the flexi-grid-channel-spacing leaf in flexi-grid-label-step grouping;
• added new flexi-ncfg-type and flexi-ncfg-6p25ghz identities;
• added new flexi-ncfg leaf to flexi-grid-label-step grouping

Removed FEC identities with no standard reference: fix #76

Replaced dmb-t with power-in-dbm: fix #77

Updated description of standard and organizational mode: fix #8

Replaced normative references to RFC6163 and RFC7698 with references to Standard Track RFCs: fix #64

Co-authored-by: sergio belotti sergio.belotti@nokia.com

[sergiobelotti]

I've approved the update but take care of my comments, in particular that one related to organizational-mode