MLK-9684-6: ARM: clk-imx6sx: add missing lvds2 clock to the clock tree

MLK-9684-6: ARM: clk-imx6sx: add missing lvds2 clock to the clock tree

We actually have lvds2 (analog clock2), an I/O clock like lvds1, in the SoC.
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.

So This patch mainly adds the lvds2 to the clock tree and fix its relationship
with pll4 accordingly.

Signed-off-by: Shengjiu Wang <shengjiu.wang@freescale.com>
(cherry picked from commit f9cfc11cf8628bd01efda611074131bfa323a120)