ARM: AM33XX: Fix typo that causes an AM duplication in CPU name.

Just fix a typo displaying the CPU info. With CONFIG_DISPLAY_INFO we see
something like AMAM335X-GP rev 0 instead of AM335X-GP rev 0.

Signed-off-by: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Reviewed-by: Javier Martinez Canillas <javier@dowhile0.org>