configs/qemu_aarch64_sbsa_defconfig: update to Linux 5.15

Increase vfat partition size for qemu-aarch64-sbsa since it now
requires more than 32M. See "Disk full" [1].

[1] https://gitlab.com/kubu93/buildroot/-/jobs/1745752049



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>