ARM: OMAP2+: Move GPMC initcall to devices.c

This will us allow to just move gpmc.c to live under drivers
in the next patch.

Note that we now also remove the omap specific check for the
initcall. That's OK as gpmc_probe() checks for the pdata
and bails out for other platforms compiled in.

Also the postcore_initcall() maybe possible to change to
just regular module_init(), but let's do that in separate
patch after the move to drivers is done.

Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>