ARM: omap2: gpmc: Mark local scoped functions static

This patch marks a bunch of functions that are local
to gpmc.c file only as static.

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>