clk: imx: Add clock configuration for ARMv7 platforms

Add CONFIG_CLK_xxx for i.MX ARMv7 platforms, and use it as build option
instead of CONFIG_SOC_xxx, the CONFIG_CLK_xxx will be selected by default
according to CONFIG_SOC_xxx.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>