MLK-10409: arm: imx: add ipg_root_clk define on i.mx7d

the ipg_root_clk is a branch of ahb_root_clk, the target interface for this
clock only has a post-div field. the output frequency is:
freq = (ahb_root_clk)/(post_dev +1). the default frequecy of this clock is 1/2
of the ahb_root_clk.

Signed-off-by: Bai Ping <b51503@freescale.com>