dt-bindings: add resets property to dw-apb-timer

The Synopsis DesignWare APB timer driver
(drivers/clocksource/dw_apb_timer_of.c) indirectly uses the resets
property of its node as it executes the function of_reset_control_get().
Make sure that this property is documented in
timer/snps,dw-apb-timer.yaml to avoid make dtbs_check warnings.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>