uboot-imx

board/synopsys: remove selection of CPU from the board

Both ARCangel4 and AXS10x are FPGA-based boards so they may have
different CPUs. For now we have only 1 option (ARC700) and we define
this as default in arch Kconfig.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>