driver: spi: cadence: remove unused variables

commit 51e67b2b (driver: spi: cadence: Add ACPI support)
replaced node_child with fwnode_child, but the old variables
were not deleted, this will cause compiler warning below, so
remove these unused variables.

drivers/spi/spi-cadence-xspi.c: In function 'cdns_xspi_of_get_plat_data':
drivers/spi/spi-cadence-xspi.c:873:29: warning: unused variable 'node_child' [-Wunused-variable]
  873 |         struct device_node *node_child;
      |                             ^~~~~~~~~~
drivers/spi/spi-cadence-xspi.c:871:29: warning: unused variable 'node_prop' [-Wunused-variable]
  871 |         struct device_node *node_prop = pdev->dev.of_node;
      |                             ^~~~~~~~~

Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>