configs/atmel_sama5d27_som1_ek_mmc_dev: increase rootfs image size

configs/atmel_sama5d27_som1_ek_mmc_dev: increase rootfs image size

The default rootfs image size is too small, causing the following
build failure:

Copying files into the device: __populate_fs: Could not allocate block in ext2 filesystem while writing file "udevd"
mkfs.ext4: Could not allocate block in ext2 filesystem while populating file system
*** Maybe you need to increase the filesystem size (BR2_TARGET_ROOTFS_EXT2_SIZE)

So we increase it to 120 MB.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/199339415

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>