dt-bindings: counter: Document stm32 quadrature encoder

Add bindings for STM32 Timer quadrature encoder.
It is a sub-node of STM32 Timer which implement the
quadratic encoder part of the hardware.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>