sniper: Power off when the power on reason is not a valid one

sniper: Power off when the power on reason is not a valid one

In most cases, userspace will attempt to power off the device with HALT instead
of POWER_OFF, which triggers a reset instead of a proper power off from the
TWL4030. Hence, it is up to the bootloader to actually turn the device off when
there is no reason to turn it on.

A reboot identified with the OMAP reboot mode bits set is acceptable, as well as
a power on reason from either the power button, USB or charger plug.

Other cases should trigger a power off. Note that for the U-Boot reset command
to take effect, we have to fill-in the OMAP reboot bits.

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