platsupport: Don't require plat/gpio.h on all plat

The Arm interface for gpio_sys_t doesn't require any platform
definitions and so we don't need to include a platform header file.

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.

platsupport: TK1 GPIO: share gpio_set_pad_mode().