ARM: OMAP2+: gpmc: Sanity check GPMC fck on probe

This prevents potential division by zero errors
if GPMC fck turns out to be zero due to faulty clock
data.

Use resource managed clk_get() API.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>