soc: renesas: rmobile-sysc: Use of_fwnode_handle() helper

Use the existing of_fwnode_handle() helper instead of open-coding the
same operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/d49b9fb20d68709b12692558aca91997e6b06fb4.1678272276.git.geert+renesas@glider.be