* spi_flash_mmap_pages needs pages array to be in internal memory.
Document and check this.
* Fix a bug that spi_flash_mmap did not allocate pages array in
internal memory.
* Minor style fixes: const-ify pages argument of spi_flash_mmap, add
spaces around operators, mark output arguments with [out].
Closes https://github.com/espressif/esp-idf/issues/2229.