ARM: OMAP2+: Allow kernel to boot even if GPMC fails to reserve memory

Currently, if the GPMC driver fails to reserve memory when probed we will
call BUG() and the kernel will not boot. Instead of calling BUG(), return
an error from probe and allow kernel to boot.

Tested on AM335x beagle bone board.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>