mtd: nand: Show reserved block in chip.erase

The "nand chip.erase" command always printed as bad blocks even in the
case of reserved blocks. Reserved blocks are used for storing bad block
tables. The patch displays "bbt reserved" when printing reserved blocks in
"nand chip.erase" command.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>