ARM: omap2+: Downgrade u-boot version warnings to debug statements

We should be able to see real issues with dmesg -l err,warn. The u-boot
revision warning should be a debug statement rather than a warning.

Cc: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Cc: Carl Philipp Klemm <philipp@uvos.xyz>
Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>