mtd: denali: detect the number of banks

Not all configurations of the Denali controller support 4 banks.  The
controller can support between 1 and 16 banks.  Detect this from the
design features register.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>