configs: p1_p2_rdb: Add the default address of vsc7385 firmware

Add the environment 'vscfw_addr' to assign a default address for
vsc7385 firmware uploading.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>