clk: mediatek: Add hardware dependency

Only propose the mediatek clock drivers on this platform, unless
build-testing.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Shunli Wang <shunli.wang@mediatek.com>
Cc: James Liao <jamesjj.liao@mediatek.com>
Cc: Erin Lo <erin.lo@mediatek.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>