soc: mediatek: pwrap: add missing check on rstc

The variable rstc is set only when the SoC PWRAP have the
PWRAP_CAP_RESET capability. Check whether rstc is set before
using it to avoid errors.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>