openrisc: use boot_command_line instead of private cmd_line

Save some pointless copying of the kernel command line and just use
boot_command_line instead. The DT code already handles CONFIG_CMDLINE,
so a separate copy is not needed.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: linux@lists.openrisc.net
Reviewed-by: Grant Likely <grant.likely@linaro.org>