dts: bindings: add generic-fem-two-ctrl-pins

This is a helper binding for radio front-end modules that have a
two-pin control interface, where one pin is used to turn on an power
amplifier (PA) for TX, and another is used to turn on a low-noise
amplifier (LNA) for RX.

Such hardware is already supported by the Bluetooth subsystem, but via
PA/LNA Kconfig settings. Since this is hardware configuration, it is
better to move this to devicetree instead.

Add a binding that makes it possible to define nodes which contain the
same information, along with a bit of extra information related to the
gain in dB of each amplifier not currently available from Kconfig.
This is similar to the existing binding for the nordic,nrf21540-fem
compatible.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Signed-off-by: Andrzej Kuros <andrzej.kuros@nordicsemi.no>