driver/mtd: Use generic timer API for FSL IFC, eLBC

Freescale's flash control driver is using architecture specific timer API
i.e. usec2ticks

Replace usec2ticks with get_timer() (generic timer API)

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>