mmap: remove implementation of mmap gpio

Instead of trying to have an implementation that covers most platforms
Leave all mmap up to platform definition. Through function pointers.

gpio_mmap_setup sets up the mmap

Within the internal gpio context struct two more function pointers now
exist mmap_read & mmap_write. They exist there so each context can its
own function for handling a write and read.

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>