dt-bindings: timer: armv7m-systick: convert to dtschema

Convert the ARMv7-M system timer bindings to DT schema format.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1644414747-22159-1-git-send-email-fabrice.gasnier@foss.st.com