ARM: omap2: gpmc: Remove unused gpmc_round_ns_to_ticks() function

This function is not used anywhere, so it's safe to remove it.
This means less code to maintain.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>