clk: meson: add missing const qualifiers on gate arrays

Reported-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20170309104154.28295-3-jbrunet@baylibre.com