clk: imx: correct i.MX7D AV PLL num/denom offset

According reference manual, i.MX7D's audio/video PLL's
num/denom register offset are 0x20/0x30, they are different
from i.MX6's audio/video PLL, correct it by introducing new
offset variables for audio/video PLL and using runtime
assignment based on PLL type.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>