dt-bindings: mfd: st,stm32-timers: Correct 'resets' property name

The correct property name for the reset binding is 'resets', not 'reset'.
Fix the name.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220719215125.1877138-1-robh@kernel.org