arm: socfpga: Unbind CPU type from board type

arm: socfpga: Unbind CPU type from board type

The CONFIG_TARGET_SOCFPGA_CYCLONE5 and CONFIG_TARGET_SOCFPGA_ARRIA5
selected both a board and a CPU. This is not correct as these macros
are supposed to select only board.

All would be good, if QTS-generated header files didn't check for
these macros exactly to determine if the platform is Cyclone V or
Arria V. Thus, for the sake of compatibility with not well fleshed
out header file generator, this patch makes these two macros into
a stub config option and introduces new CONFIG_TARGET_SOCFPGA_CYCLONE5_SOCDK
and CONFIG_TARGET_SOCFPGA_ARRIA5_SOCDK targets, which select the
previous stub config option.

The result is that compatibility with QTS is preserved and the new
CONFIG_TARGET_* select actual target boards.

Signed-off-by: Marek Vasut <marex@denx.de>