clk: gpio-gate: Don't export __init functions

This function is marked as __init, so exposing it to modules
doesn't make any sense and it isn't used by modules anyway.

drivers/clk/clk-gpio-gate.c:192:13: warning: symbol 'of_gpio_gate_clk_setup' was not declared. Should it be static?

Cc: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>