reboot: hide from sysfs not applicable settings

Not all the reboot settings from both the kernel command line or sysfs
interface are available to all platforms.

Filter out reboot_type and reboot_force which are x86 only, and also
remove reboot_cpu on kernels without SMP support.

This saves some space, and avoid confusing the user with settings which
will have no effect.

Link: https://lkml.kernel.org/r/20201130173717.198952-3-mcroce@linux.microsoft.com
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>