staging: mt7621-dts: update nor-flash info for gnubee1

The GNUBEE has 32MB flash, so set partitions accordingly.
Also remove "m25p,chunked-io" which isn't documented or
used anywhere (outside of freewrt).

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>