mx6: clock: Introduce disable_ipu_clock()

Introduce disable_ipu_clock(). This is done in preparation for
configuring the NoC registers on i.MX6QP in SPL.

Afer the NoC registers are set the IPU clocks can be disabled.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>