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: NDinh Nguyen <dinguyen@opensource.altera.com>

arch/arm/mach-socfpga/Kconfig

@@ -2,9 +2,14 @@ if ARCH_SOCFPGA
 
 config TARGET_SOCFPGA_ARRIA5
 	bool
+	select TARGET_SOCFPGA_GEN5
 
 config TARGET_SOCFPGA_CYCLONE5
 	bool
+	select TARGET_SOCFPGA_GEN5
+
+config TARGET_SOCFPGA_GEN5
+	bool
 
 choice
 	prompt "Altera SOCFPGA board select"