ARM i.MX6: rename PLLs according to datasheet

In recent reference manuals the PLLs were renumbered. PLL8 now is
PLL6 and vice versa. Change the code according to the reference
manual to avoid confusion.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>