sysreset: socfpga: Use parent device for reading base address

This driver is a child of the rstmgr driver, both of which share the
same devicetree node. As a result, passing the child's udevice pointer
to dev_read_addr_ptr results in a failure of reading the #address-cells
property. Use the parent udevice pointer instead.

Signed-off-by: Paweł Anikiel <pan@semihalf.com>
Reviewed-by: Simon Glass <sjg@chromium.org>