imx8ulp: ddr: Fix DDR frequency request issue

imx8ulp: ddr: Fix DDR frequency request issue

After acking the requested frequency, should wait the ack bit clear
by DDR controller and check the DFS interrupt for next request polling.
Otherwise, the next polling of request bit will get previous value
that DDR controller have not cleared it, so a wrong request frequency
is used.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>