clk: meson: a1: Staticize rtc clk

clk: meson: a1: Staticize rtc clk

Sparse rightly complains that this symbol is supposed to be static.

Cc: Jian Hu <jian.hu@amlogic.com>
Cc: Dmitry Rokosov <ddrokosov@sberdevices.ru>
Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Fixes: 84af914404db ("clk: meson: a1: add Amlogic A1 Peripherals clock controller driver")
Link: https://lore.kernel.org/r/20230612182332.371003-1-sboyd@kernel.org
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>