mtd: rawnand: denali: remove DENALI_NR_BANKS macro

Use the runtime-detected denali->nbanks instead of hard-coded
DENALI_NR_BANKS (=4).

The actual number of banks depends on the IP configuration, and
can be less than DENALI_NR_BANKS. It is pointless to touch
registers of unsupported banks.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>