cc2540: Add support for CC2540 SoC

This patch adds the initial support for TI's CC2540 System-on-Chip.
It basically adds the cc2540.h header which defines some macros to
access Special Function Registers (SFR) and the interrupt numbers
from CC2540 SoC. It also adds the Makefile.cc2540 with the compiling
directives in order to build platforms based on CC2540 SoC.

These initial files were written base on cpu/cc253x files.

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

This patch adds support for clock module to CC2540 SoC.

Since clock_wait(), clock_delay_usec() and clock_fine_max() are
only used in platform specific code, we don't implement them by
now. We might support them when they are needed.

The deprecated function clock_fine() is lacking a proper
documentation so we define it as an empty function. Some other
SoC clock implementations don't even define this function.

Moreover, clock_set_seconds(), seems not to be used by anyone in
Contiki source tree so we don't implement it.

This patch also creates the isr.h header file. All interrupt
service routine prototypes must be declared in this file. This
header should be included by the file which defines the main()
function, otherwise SoC ISRs will not be registered.

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>
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>
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>