clk: imx: Explicitly include bits.h

It is better to explicitly include the required header file rather
then get it through some recursive include.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>