nvs: support sector_size of 64KB

Allows NVS to work with flash device configured to
use only 64KB block erase. Due to how addresses
are encoded internally in NVS, 64KB is the maximum
sector size. Add a test for this during mount.

Add a native_sim unit test case for 64kb erase block size

Signed-off-by: Mike J. Chen <mjchen@google.com>