clk: imx: add scu clock common part

Add SCU clock common part which will be used by client clock drivers.
SCU clocks are totally different from the legacy clocks (No much
legacy things can be reused), it's using a firmware interface now based
on SCU protocol. So a new configuration option CONFIG_MXC_CLK_SCU is added.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
[sboyd@kernel.org: Mark ccm_ipc_handle static]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>