memory: omap-gpmc: make gpmc_clk_ticks_to_ns() static

We get 1 warning when build kernel with W=1:
drivers/memory/omap-gpmc.c:354:14: warning: no previous prototype for 'gpmc_clk_ticks_to_ns' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is declared
and don't need a declaration, but can be made static.
so this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Roger Quadros <rogerq@ti.com>