blackfin: defconfigs: add MTD_SPI_NOR (new dependency for M25P80)

These defconfigs contain the CONFIG_M25P80 symbol, which is now
dependent on the MTD_SPI_NOR symbol. Add CONFIG_MTD_SPI_NOR to the
relevant defconfigs.

At the same time, drop the now-nonexistent CONFIG_MTD_CHAR symbol.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Steven Miao <realmz6@gmail.com>
Cc: adi-buildroot-devel@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org