mfd: rk808: Always use poweroff when requested

With the device tree property "rockchip,system-power-controller" we
explicitly request to use this PMIC to power off the system. So always
register our poweroff function, even if some other handler (probably
PSCI poweroff) was registered before.

This does tend to reveal a warning on shutdown due to the Rockchip I2C
driver not implementing an atomic transfer method, however since the
write to DEV_OFF takes effect immediately the I2C completion interrupt
is moot anyway, and as the very last thing written to the console it is
only visible to users going out of their way to capture serial output.

Signed-off-by: Soeren Moch <smoch@web.de>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
[ rm: note potential warning in commit message ]
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>