reboot: export symbol 'reboot_mode'

Some drivers like Qualcomm pm8941-pwrkey need to access 'reboot_mode'
for triggering reboot between cold and warm mode.  Export the symbol, so
that drivers built as module can still access the symbol.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Link: https://lore.kernel.org/r/20210714095850.27185-2-shawn.guo@linaro.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>