dt-bindings: counter: Adjust dt-bindings for STM32 lptimer move

The STM32 LP Timer counter driver now resides under the Counter
subsystem. This patch adjusts dt-bindings to account for the STM32
lptimer driver move.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@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>