dt-bindings: mfd/syscon: Add resets property

Simple syscon devices may require deassertion of a reset signal in order
to access their register set. This change adds the `resets` property from
reset.yaml#/properties/resets (referenced through core.yaml), specifying
a maxItems of 1 for a single (optional) reset descriptor.

This will allow a future change to the syscon driver to implement reset
control.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230105005010.124948-2-jk@codeconstruct.com.au