Add AS923-925 plan for TTN Australia

[htdvisser]

Summary

As far as I understood, the AS923-925 plan for TTN Australia is the same as regular AS923-925 but with a higher maximum EIRP.

Changes

• Copy max eirp from DefaultMaxEIRP in https://github.com/TheThingsNetwork/lorawan-stack/blob/master/pkg/band/au_915_928.go#L153
• Update index

[johanstokking]

@ama9910