at86rf2xx: (at86rf212b) Add support for Chinese 780 MHz band

[jnohlgard]

IEEE 802.15.4c-2009

Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (WPANs)
Amendment 2: Alternative Physical Layer Extension to support one or more of the Chinese 314–316 MHz, 430–434 MHz, and 779–787 MHz bands

channel page 5, channels 0-3

Requires some messing around in the CC_CTRL_0, CC_CTRL_1 registers (CC_BAND and CC_NUMBER)

Just writing down the idea here, I don't have a use case currently.

[biboc]

True! Add it to your PR you talked about in #3452

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