ARM: socfpga: move board select into mach-socfpga/Kconfig

Switch to a more standard way of board select; put the SoC select
into arch/arm/Kconfig and move the board select menu under
arch/arm/mach-socfpga/Kconfig.

Also, consolidate SYS_BOARD, SYS_VENDOR, SYS_SOC, SYS_CONFIG_NAME.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/configs/socfpga_arria5_defconfig b/configs/socfpga_arria5_defconfig
index 52032e5..d658a50 100644
--- a/configs/socfpga_arria5_defconfig
+++ b/configs/socfpga_arria5_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SPL=y
 CONFIG_ARM=y
+CONFIG_ARCH_SOCFPGA=y
 CONFIG_TARGET_SOCFPGA_ARRIA5=y
 CONFIG_OF_CONTROL=y
 CONFIG_DEFAULT_DEVICE_TREE="socfpga_arria5_socdk"