ARM: OMAP: split OMAP1, OMAP2+ RNG device registration

Move the OMAP1-specific RNG device creation off to mach-omap1/devices.c,
and create a omap_device-backed registration function for OMAP2+ devices
in mach-omap2/devices.c.

As a nice side-benefit, we can also get rid of
arch/arm/plat-omap/devices.c, thanks to some recent changes from Tony.

One change from the previous behavior is that the RNG devices are now
registered unconditionally.  This should allow the RNG drivers to be
loaded as modules, even if the original kernel was not built that way.

Signed-off-by: Paul Walmsley <paul@pwsan.com>