ARM: OMAP2+: drop omap_device_register() helper

The omap_device_register() helper is unused since commit 82f12e64
("ARM: OMAP2+: Drop legacy init for sdma") and can be removed.

Signed-off-by: Johan Hovold <johan@kernel.org>
Message-Id: <20220303180014.2639-2-johan@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>