tests: ext2: reduce ram disk size for small.overlay

- Reduces the ram disc sector count from 400 to 220.
- Saves about 90KB of RAM. All tests still passed.
- Allows platforms with less RAM to run
  the ramdisc_small.overlay test.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>

Author: butok

tests/subsys/fs/ext2/ramdisk_small.overlay

@@ -9,6 +9,6 @@
 		compatible = "zephyr,ram-disk";
 		disk-name = "RAM";
 		sector-size = <512>;
-		sector-count = <400>;
+		sector-count = <220>;
 	};
 };