ENGR00220370 [MX6]Fix BUS freq suspend/resume fail in low bus mode

ENGR00220370 [MX6]Fix BUS freq suspend/resume fail in low bus mode

1. BUS freq's set low bus setpoint using delat work, which
didn't have mutex lock, so in some scenarios, set high bus
freq function can be called at the same time, we need to move
mutex lock into these two routine;

2. Using pm notify to make sure bus freq set to high setpoint
before supend and restore after resume.

3. Clear build warning.

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