mtd: nand: denali: move multi device fixup code to a helper function

Collect multi NAND fixups into a helper function instead of
scattering them in denali_init().

I am rewording the comment block to clearly explain what is called
"multi device".

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