Revert "Remove unused LPSPI I/O helpers"

This reverts commit 0cb3b8d.

src/common/lpspi.rs

@@ -1518,6 +1518,15 @@ where
     }
 }
 
+/// Transmits dummy values.
+struct TransmitDummies;
+
+impl TransmitData for TransmitDummies {
+    fn next_word(&mut self, _: BitOrder) -> u32 {
+        u32::MAX
+    }
+}
+
 /// Receive data into a buffer.
 struct ReceiveBuffer<'a, W> {
     /// The write position.
@@ -1582,6 +1591,13 @@ where
     }
 }
 
+/// Receive dummy data.
+struct ReceiveDummies;
+
+impl ReceiveData for ReceiveDummies {
+    fn next_word(&mut self, _: u32) {}
+}
+
 /// Computes how may Ws fit inside a LPSPI word.
 const fn per_word<W: Word>() -> usize {
     core::mem::size_of::<u32>() / core::mem::size_of::<W>()