dt-bindings: mfd: lp875xx: Add optional reset GPIO

Document the LP8756x-Q1 and LP87524-Q1 ICs reset pin.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>