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>
diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index d3ef58b..6a77b8f 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -4,6 +4,9 @@
 config SYS_ARCH
 	default "arc"
 
+config SYS_CPU
+	default "arc700"
+
 choice
 	prompt "Target select"