Add nRF52840 support

[knopserl]

The nRF52840 has plenty of addvantages over the nRF52832:
Full BLE 5.0 with long range support, highter RF output, 4 x RAM and 2 x Flash, 1.7-5.5V supply, HW encryption, USB interface etc. ect.
Meanwhile a couple of cheap modules are available eg from RF-star, holyiot, EBYTE, Nordic and many others. I'm already working with some of these modules but to use thema with Arduino, it would be required to have also a board definition for the nRF52840.
We can use the similar pinout as the nRF52832 but the other parameters have to be define correct.
So no need to provide all mappings for all variants just a generic nRF52840 MyBoardNRF5 please.