arm: socfpga: introduce TARGET_SOCFPGA_GEN5 config property

In order to re-use as much Cyclone5 and Arria5 code as possible to support
the Arria10 platform, we need to wrap some of the code with #ifdef's. By
adding CONFIG_TARGET_SOCFPGA_GEN5, we can shorten the check by not having to check
for both AV || AV.

Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>