Split drv/digital_io.h to drv/gio*.h and drv/port.h

Split drv/digital_io.h to drv/gio*.h and drv/port.h

File digital_io.h mixed two concepts - digital IOs and so called ports.
Ports can be used to access digital IOs but also SPI and ADC. We
therefore spit this file into two independent one.

File gio.h defines access to digital IO on the MCU. The definition of
pins will be provided per target. Applications do not access this
definition directly but via two files: pin names are accessed via
gio_pin_names.h and the definition and related functions via
gio_pin_tab.h.

File port.h defines access to ports.

This commit only changes the interface (.h files). Target specific port
and pin definitions and changes in other files are in subsequent
commits.