dts: riscv: add Generic FSK node

The Generic FSK controller enables radio operation
using a custom GFSK/GMSK or MSK modulation format
achieved by programming a set of PHY variables such
as BT product, modulation index and modulation filter
co-efficients (such that max frequency deviation
<= 500kHz). Generic FSK mode also offers a highly
configurable packet structure, variable bit rate
transmission and reception, some limited packet
(header) processing, and interface to a RAM-based
Packet Buffer.

Signed-off-by: Alex Porosanu <alexandru.porosanu@nxp.com>