Implement mmap for BCH and FTL

After https://github.com/apache/nuttx/pull/8000, mmap/munmap is the first-class interface, since block/mtd(e.g. RAM disk or XIP flash) device may support mmap too, It could be useful to:

1. Add mmap callback to block_operations and mtd_dev_s:
https://github.com/apache/nuttx/blob/master/include/nuttx/fs/fs.h#L260-L274
https://github.com/apache/nuttx/blob/master/include/nuttx/mtd/mtd.h#L141-L187
2. Forward mmap from file_operations to block_operations to mtd_dev_s:
https://github.com/apache/nuttx/blob/master/drivers/bch/bchdev_driver.c#L81
https://github.com/apache/nuttx/blob/master/drivers/mtd/ftl.c#L106-L117