boards: arm: Renamed efr32bg22_sltb010a and updated docs

Vendor's P/Ns are volatile and there isn't one yet assigned to the new
BG27 board. Thunderboard-style boards are now all named after the number on
printed on their PCBs.

Signed-off-by: Krzysztof Boronski <kboronski@antmicro.com>