stmmac: dwmac-loongson: Add architecture dependency

Only present the DWMAC_LOONGSON option on architectures where it can
actually be used.

This follows the same logic as the DWMAC_INTEL option.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>