clk: meson: make shipped controller configurable

Add the necessary bits so unnecessary amlogic clock controllers can be
compiled out. This allows to save a few kB when necessary.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20200828154735.435374-1-jbrunet@baylibre.com