arm/dts: OMAP: Remove bootargs node from board files

Since 3.2, the CONFIG_ARM_ATAG_DTB_COMPAT config allows
an old bootloader to still use ATAG to provide cmdline.

Remove chosen/bootargs from the DTS board files.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>