clk: berlin: Silence sparse warning

drivers/clk/berlin/berlin2-pll.c:94:12: warning: symbol 'berlin2_pll_register' was not declared. Should it be static?

Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>