ENGR00221643 [MX6]Fix race condition of pfd 400 usecount

ENGR00221643 [MX6]Fix race condition of pfd 400 usecount

We can't modify the usecount of pfd 400M clock when ARM freq
is changed, as when the children of pfd 400M do clock enable/disable,
they will also modify this usecount, these two modification is
out of same lock protection. And this wrong usecount may lead to
pfd 400M or pll2 disabled accidently, and it will cause system hang!

Signed-off-by: Anson Huang <b20788@freescale.com>