platsupport: TK1 Mux: Add mux config for using UARTB pins as GPIOs

* This new mux feature will steal 2 of UART-B's pinouts (RTS and CTS)
and use them as GPIOs for debugging.

platsupport: TK1 Mux: Add PA3 to mux driver

platsupport: TK1 mux: move the mux pingroup tables into a new source file

platsupport: TK1: Add a MUX driver and define mux pinout configs

This driver properly support pin output drivers and drive strengths as
well as the different output drivers supported by each pin - push-pull,
open-drain, tristate, pullup and pulldown.

Furthermore, it also adds the code to determine the strength of the slew rate
at which these signals are driven out. This feature was originally added to
investigate whether or not the I2C0 lines could be forced up to 3.3v from 1.8v.

fix licenses

libplatsupport: Refactor tk1 gpio/mux code

This commit refactors the mux and gpio code of the tk1 in the following ways
* Put GPIO code in gpio.c, not mux.c
* Bring back platform generic mux_sys_init and gpio_sys_init functions
* Make GPIO depend upon a mux without the mux initializing the GPIO
* Remove globals and use the provided 'priv' parts of the 'sys' structs
* Require gpio_sys_t be passed to 'gpio_*' functions instead of using
global variables
* Remove circular dependencies in headers and '#ifndef CONFIG_PLAT_TK1's from
generic arm headers

Integrating changes from DARPA repo to trunk

arm/tk1: add serial and timer drivers

arm/tk1: add serial and timer drivers