Platform: Ensure flash driver is initialised

Adds a call to the flash driver initialisation function in the dummy NV
counter implementation before any other flash driver functions are
called. Previously, it relied on the flash driver being already
initialised, but this is not necessarily always the case.

Change-Id: I061d3ae61c62704e652e161c7619d0b882c661f7
Signed-off-by: Jamie Fox <jamie.fox@arm.com>