buildroot

arch/arm: default to Cortex-A53 for AArch64

Since we re-organised the list of cores (in 52d500aa35) and introduced
some new cores (in e9960da6ec, d632d9e5a9, 6317a199ec), the default for
AArch64 was accidently changed from A53 to A35.

So, restore the default to A53 for AArch64.

Reported-by: daggs <daggs@gmx.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: daggs <daggs@gmx.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>