dt-bindings: net: ath10k: add new dt entry to identify external FEM

This adds new dt entry ext-fem-name, it is used by ath10k driver
to select correct timing parameters and configure it in target wifi hardware.
The Front End Module(FEM) normally includes tx power amplifier(PA) and
rx low noise amplifier(LNA). The default timing parameters like tx end to
PA off timing values were fine tuned for internal FEM used in reference
design. And these timing values can not be same if ODM modifies hardware
design with different external FEM. This DT entry helps to choose correct
timing values in driver if different external FEM hardware used.

Signed-off-by: Bhagavathi Perumal S <bperumal@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>