st7789: add Bus interface to accomodate both SPI and parallel connections to display

Signed-off-by: deadprogram <ron@hybridgroup.com>

st7789/bus.go

@@ -0,0 +1,8 @@
+package st7789
+
+// Bus is the interface that wraps the basic Tx and Transfer methods
+// for communication buses like SPI or Parallel.
+type Bus interface {
+	Tx(w, r []byte) error
+	Transfer(w byte) (byte, error)
+}