script: add ram and rom total size to json file

It's easier and safer to parse json file rather than extract
total size from stdout

Now the .json file has two keys:
{
    "symbols": {
       "children": [
           ...
	],
       "identifier": ":",
       "name": "root",
       "size": 2220
    },
    "total_size": 34272
}

Signed-off-by: Jingru Wang <jingru@synopsys.com>