soc: amlogic: socinfo: build for specific arch

The MX driver only supports 32-bit ARM SoCs and the GX driver only
supports 64-bit SoCs.  Only build for the right architecture.

Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>