OMAP: Store reboot mode in scratchpad on OMAP34xx

The reboot mode can be communicated to a bootloader (or the
kernel itself) with a scratchpad register. This functionality
is especially useful, if userspace is allowed to change
the reboot mode.

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>