dt-bindings: clock: add imx7ulp clock binding doc

i.MX7ULP Clock functions are under joint control of the System
Clock Generation (SCG) modules, Peripheral Clock Control (PCC)
modules, and Core Mode Controller (CMC)1 blocks

Note IMX7ULP has two clock domains: M4 and A7. This binding doc
is only for A7 clock domain.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Anson Huang <Anson.Huang@nxp.com>
Cc: Bai Ping <ping.bai@nxp.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>