Add code for talking to the Teensy's SPI peripheral

Change-Id: I3ddaa616e3f84fa74a2e7ab68726d5cb5cdbc9de
diff --git a/motors/peripheral/BUILD b/motors/peripheral/BUILD
index 2c71973..c8b63e9 100644
--- a/motors/peripheral/BUILD
+++ b/motors/peripheral/BUILD
@@ -90,3 +90,17 @@
         "//motors/core",
     ],
 )
+
+cc_library(
+    name = "spi",
+    srcs = ["spi.cc"],
+    hdrs = ["spi.h"],
+    restricted_to = mcu_cpus,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":uart_buffer",
+        "//motors:util",
+        "//motors/core",
+        "//third_party/GSL",
+    ],
+)