clk: imx: Move 'imx6sl_set_wait_clk()'s prototype out to accessible header

Fixes the following W=1 kernel build warning(s):

 drivers/clk/imx/clk-imx6sl.c:156:6: warning: no previous prototype for ‘imx6sl_set_wait_clk’ [-Wmissing-prototypes]

Cc: Russell King <linux@armlinux.org.uk>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Cc: Ahmad Fatoum <a.fatoum@pengutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>