stm32cube: extend public SetFlashLatency API to all families

The LL_SetFlashLatency public API is now defined for families
beyond G4, L4 and L5. In most case the private function has been
made public, in one case (H7) it's built on top of existing ones.

Signed-off-by: Giancarlo Stasi <giancarlo.stasi.co@gmail.com>