AArch64: Set TARGET_2ND_CPU_VARIANT for Arm64 to cortex-a15

A sensible default for the 32bit CPU_VARIANT of an ARM64 build is
cortex-a15. Please note that the AArch32 execution state of ARMv8
is a superset of ARMv7.

Change-Id: Id2b655172750e04609ae9ba22d621fe83cd69b1a
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>