firmware: xilinx: make firmware_debugfs_root static

Fix the following sparse warning:

drivers/firmware/xilinx/zynqmp-debug.c:38:15: warning: symbol
'firmware_debugfs_root' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200415084311.24857-1-yanaijie@huawei.comSigned-off-by: Michal Simek <michal.simek@xilinx.com>

drivers/firmware/xilinx/zynqmp-debug.c

@@ -35,7 +35,7 @@ static struct pm_api_info pm_api_list[] = {
 	PM_API(PM_QUERY_DATA),
 };
 
-struct dentry *firmware_debugfs_root;
+static struct dentry *firmware_debugfs_root;
 
 /**
  * zynqmp_pm_argument_value() - Extract argument value from a PM-API request