dt-bindings: clock: Convert imx7ulp clock to json-schema

Convert the i.MX7ULP clock binding to DT schema format using json-schema,
the original binding doc is actually for two clock modules(SCG and PCC),
so split it to two binding docs, and the MPLL(mipi PLL) is NOT supposed
to be in clock module, so remove it from binding doc as well.

Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>