clk: imx: imx7d: add the snvs clock

According to the i.MX7D Reference Manual,
SNVS block has a clock gate, accessing SNVS block
would need this clock gate to be enabled, add it
into clock tree so that SNVS module driver can
operate this clock gate.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>