dt-bindings: timer: Convert st,spear-timer to DT schema

Convert the ST SPEAr Timer binding to DT schema format. It's a
straight-forward conversion.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250506022326.2589389-1-robh@kernel.org


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>