ARC: [plat-axs10x] auto-select AXS101 or AXS103 given the ISA config

AXS10x platform has two flavours:
 - AXS101 (ARC700 cpu, ARCompact ISA)
 - AXS103 (ARC HS38x cpu, ARCv2 ISA)

So depending on the ISA support configured, pre-select the AXS10x
variant.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>