arch: nios2: rename 'altr,gpio-bank-width' -> 'altr,ngpio'

There is no more 'altr,gpio-bank-width' in the 'altr,pio-1.0' driver.
There is a 'altr,ngpio' which is  what the property wants to configure.

This change updates all occurrences of 'altr,gpio-bank-width' to
'altr,ngpio'.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>