cmd_bdinfo: check for CONFIG_ARC instead of CONFIG_ARC700

For all flavours of ARC we execute the same code in "bdinfo" so we may
safely check for CONFIG_ARC. This is especially important since we're
about to add more types of ARC so existing check won't work in all
cases.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Tom Rini <trini@ti.com>