clk: meson: enable building as modules

Make it possible to build all clk drivers as modules, but default
remains built-in.

No functional changes.

Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20201118191405.36798-1-khilman@baylibre.com