cc2540mini: Initial support for CC2540 mini development kit

This patch adds the initial files for supporting CC2540 mini
development kit from TI. Since clock module and etimer library
is already supported by the SoC, we initialize timer and process
support for this platform.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
cc2540: rtimer module initial support

Since the SoC dependent file rtimer-arch.h is required to build
Contiki, we add the inital support for rtimer module. By now,
we use the system clock module to implement the rtimer module.
Nevertheless, the rtimer module will be properly support in future.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
cc2540: Add mtarch.h header

This patch adds the mtarch.h header which defines a dummy struct
mtarch_thread in order we are able to build Contiki for cc2540mini
platform.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
cc2540mini: Add LED support

This patch adds the LED support for cc2540mini platform. The
cc2540mini platform has only two LEDs (green and red).

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>