dt-bindings: firmware: imx-scu: new binding to parse clocks from device tree

There's a few limitations on the original one cell clock binding
(#clock-cells = <1>) that we have to define some SW clock IDs for device
tree to reference. This may cause troubles if we want to use common
clock IDs for multi platforms support when the clock of those platforms
are mostly the same.
e.g. Current clock IDs name are defined with SS prefix.

However the device may reside in different SS across CPUs, that means the
SS prefix may not valid anymore for a new SoC. Furthermore, the device
availability of those clocks may also vary a bit.

For such situation, we want to eliminate the using of SW Clock IDs and
change to use a more close to HW one instead.
For SCU clocks usage, only two params required: Resource id + Clock Type.
Both parameters are platform independent. So we could use two cells binding
to pass those parameters,

Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: devicetree@vger.kernel.org
Acked-by: Shawn Guo <shawnguo@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>