lm4f: Add lm4f support files copied from lm3s

Create lm4f code infrastructure from the lm3s infrastructure.

As far as the interrupt table is concerned, don't create an irq.yaml. Just
include the LM3S nvic.h. The LM3S vector table seems to be compatible with the
LM4F

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

include/libopencm3/lm3s/irq.yaml

@@ -1,3 +1,8 @@
+# Although this says LM3S, the interrupt table applies to the LM4F as well
+# Some interrupt vectores marked as reserved in LM3S are used in LM4F, and some
+# vectors in LM3S are marked reserved for LM4F. However, the common vectors are
+# identical, and we can safely use the same interrupt table. Reserved vectors
+# will never be triggered, so having them is perfectly safe.
 includeguard: LIBOPENCM3_LM3S_NVIC_H
 partname_humanreadable: LM3S series
 partname_doxygen: LM3S