dts: bindings: add modem binding for RV32M1 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

This binding describes the devices and its associated properties.

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