rockchip: rk356x: Enable poweroff command

With PMIC_RK8XX, SYSRESET and CMD_POWEROFF options enabled it is
possible to power down a board using the poweroff command and turn the
board back on using a power button.

Enable the poweroff command on RK356x boards that have a button wired
to PMIC pwron.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>