cmd_bootm.c: Only pass BOOTM_STATE_OS_CMDLINE on PowerPC/MIPS

In 5c427e49 we pass BOOTM_STATE_OS_CMDLINE as part of the bootm states to
run, on all arches.  However, this is only valid / useful on PowerPC and
MIPS, and causes a problem on ARM where we specifically do not use it.
Rather than make this state fake pass like we do for GO on some arches
(which need updating to use the GO state), we should just not pass
CMDLINE except when it may be used, like before.

Tested-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Tom Rini <trini@ti.com>