[MTD] Make init_impa7 static

The function init_impa7 does not need to be global, and
this patch makes it static by adding the needed keyword
to drivers/mtd/maps/impa7.c.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>