clk: imx7d: Fix the DDR PLL enable bit

Commit ad149724 ("clk: imx7d: Fix the powerdown bit location
of PLL DDR") used the incorrect bit for the IMX_PLLV3_DDR_IMX7 case.

Fix it accordingly to avoid a kernel hang.

Reported-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>