DVFS: Remove notifications from DVFS HAL

DVFS HAL module is not required to handle notifications. The clock
drivers are responsible for saving the H/W state during suspend/resume
operations. There is nothing to do with the DVFS context for suspend/
resume.

Change-Id: I2d401d80a91aa174c72938d017897ef1119324f2
Signed-off-by: Jim Quigley <jim.quigley@arm.com>