Add 'gpio' command and enable it by default

Add 'gpio' command and enable it by default

This adds 'gpio' command, which allows to manipulate GPIO subsystem:
 - change direction
 - change output value
 - read input value (useful for scripts)
 - enable/disable JTAG
 - manage special/dedicated GPIO functions
 - list current configuration and available functions

For QCA95xx series, this allows full management of GPIO output and
input functionalities (mux). For AR933x, due to its capabilities,
function management is more simple and allows only enabling or
disabling selected bits in GPIO_FUNCTION_{1,2} registers.

Example commands:
 - list GPIO configuration:   gpio l
 - list available functions:  gpio l f
 - set GPIO10 output to high: gpio s 10
 - get GPIO10 input value:    gpio i 10