Add a NOR trait to encode that word clearing is supported

This is to encode MCU flashes that have ECC, where
it is common that only a complete clear of the
word is allowed. This is a more restrictive
version of `MultiwriteNorFlash` as it cannot
encode flashes with ECC support.

src/nor_flash.rs

@@ -178,6 +178,15 @@ impl<T: NorFlash> NorFlash for &mut T {
 	}
 }
 
+/// Marker trait for NorFlash relaxing the restrictions on `write`.
+///
+/// Writes to the same word twice are now allowed as long as the second write is all 0s.
+/// This is common for flashes that have ECC, where after the first write only a complete clear of
+/// the word is allowed after initial write.
+pub trait WordclearNorFlash: NorFlash {}
+
+impl<T> WordclearNorFlash for T where T: MultiwriteNorFlash {}
+
 /// Marker trait for NorFlash relaxing the restrictions on `write`.
 ///
 /// Writes to the same word twice are now allowed. The result is the logical AND of the