mtd: nand: denali: clean up comments

This driver explains too much about what is apparent from the code.

Comments around basic APIs such as init_completion(), spin_lock_init(),
etc. seem unneeded lessons to kernel developers.
(With those comments dropped, denali_drv_init() is small enough,
so it has been merged into the probe function.)

Also, NAND driver developers should know the NAND init procedure, so
there is no need to explain nand_scan_ident/tail.

I removed FSF's address from the license blocks, and added simple
comments to struct members.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>