clk: imx7d: Fix the powerdown bit location of PLL DDR

According to the MX7D Reference Manual the powerdown bit of
CCM_ANALOG_PLL_DDRn register is bit 20, so fix it accordingly.

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