clk: imx6sx: add missing lvds2 clock to the clock tree

i.MX6SX has lvds2 (analog clock2), an I/O clock like lvds1.
And this lvds2, along with lvds1, can be used to provide
external clock source to the internal pll, such as pll4_audio
and pll5_video.

This patch mainly adds the lvds2 to the clock tree and fix its
relationship with pll accordingly.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>