MLK-11630 ARM: imx: register imx_clk_gate3 as fixed_factor when M4 is enabled

As clock root does NOT have domain control on i.MX7D,
and the clock root gate does NOT consume much power,
so it is acceptable to leave clock root always enabled
when M4 is enabled, this is suggested by design team,
otherwise, need to use shared memory to control clk
root between A7 and M4, which is NOT friendly.

Signed-off-by: Anson Huang <b20788@freescale.com>