boards: silabs: Add xg24_rb4186c board Add support for xG24-RB4186C, a 10 dBm radio board using the EFR32MG24 SoC. Pinout is mostly identical to RB4187C, so share devicetree definitions between them. Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>