omap3: Reboot mode support

Reboot mode is written in scratchpad memory before reboot in the form of a
single char, that is the first letter of the reboot mode string as passed to the
reboot function.

This mechanism is supported on OMAP3 both my the upstream kernel and by various
TI kernels.

It is up to each board to make use of this mechanism or not.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Reviewed-by: Tom Rini <trini@konsulko.com>