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>