ARM: 5906/1: arm: change command_line to cmd_line

drivers/of/fdt expects a cmd_line symbol, while arm uses command_line.
Change to the former, so that we can eventually share with the fdt
code.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>