clk: imx: composite-8m: add imx8m_clk_hw_composite_core

There are several clock slices, current composite code
only support bus/ip clock slices, it could not support core
slice.

So introduce a new API imx8m_clk_hw_composite_core to support
core slice. To core slice, post divider with 3 bits width and
no pre divider. Other fields are same as bus/ip slices.

Add a flag IMX_COMPOSITE_CORE for the usecase.

Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>