drivers: cache: Fix circular dependency between cache API header files

When `CONFIG_EXTERNAL_CACHE=y`, the `zephyr/cache.h` header file
includes `zephyr/drivers/cache.h`, which then includes `zephyr/cache.h`
again.

This commit fixes the above circular dependency by removing the
unneeded `zephyr/cache.h` inclusion from the `zephyr/drivers/cache.h`.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>