arm64: defconfig: Increase MMC_BLOCK_MINORS to 16

Increase the block minors from the default 8 to 16. The db410c board
by default has eMMC rootfs on the 10th partition.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>