Platform: CS300: Fix CP flag settings

Moved the coprocessor related flags to a new cmake target
so it can be used for S/NS targets as well.
Changed the flash_drivers to interface lib so the CP flags
don't have to be added explicitly.

Change-Id: I4aa5e0586dc9408c0b93212e3f3d4dec20ca4b5e
Signed-off-by: Bence Balogh <bence.balogh@arm.com>
4 files changed