ARM: at91: add MTD_SPI_NOR (new dependency for M25P80)

This defconfig contains the CONFIG_M25P80 symbol, which is now
dependent on the MTD_SPI_NOR symbol. Add CONFIG_MTD_SPI_NOR to satisfy
the new dependency.

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

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>