configs/stm32mp157c_dk2: disable ext4 metadata_csum option

configs/stm32mp157c_dk2: disable ext4 metadata_csum option

The U-Boot ext4 write support doesn't work with the metadata_csum
option, and the stm32mp157 U-Boot defconfig uses an environment stored
in an ext4 filesystem, so we must create the ext4 root filesystem
without the metadata_csum option enabled.

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
[Thomas: only disable the metadata_csum option, the 64bit and
dir_index options can be kept enabled]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>