qemu/aarch64-virt: Emulate cortex-a53 in qemu to match Buildroot config

qemu_aarch64_virt_defconfig (implicitly) specifies cortex-a53, so adjust the
QEMU command line to also emulate a a53 instead of a57.

Also adjust the defconfig to explicitly specify a53 for consistency/clarity.

Signed-off-by: Gerome Burlats <gerome.burlats@smile.fr>
Cc: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>