mtd: nand: denali: fix comment of denali_nand_info::flash_mem

The same comment "Mapped io reg base address" for flash_reg and
flash_mem probably due to the mistake of copy-paste work.
Of course, the latter is not the register base address.

Reword the comments using the terminology in the Denali User's Guide.

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