ARM: OMAP2+: Make GPMC skip disabled devices

Currently, child nodes of the gpmc node are iterated and probed
regardless of their 'status' property. This means adding 'status =
"disabled";' has no effect.

This patch changes the iteration to only probe nodes marked as
available.

Signed-off-by: Guido Martínez <guido@vanguardiasur.com.ar>
Tested-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>