clk: oxnas: Add hardware dependencies

The clk-oxnas driver is specific to its architecture, so do not
propose it on other architectures, unless build-testing.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20160707091844.196a7930@endymion