rk8xx_poweroff: enable power-cycling on support hardware

Previously, the function would return early if RB_POWERCYCLE was
specified without RB_POWEROFF. Those flags are exclusive at the moment,
that is, they are never set together.

Søren Schmidt (sos) uses a similar but extended patch locally.

(cherry picked from commit 8b408fc6f2f6dab2bfc200edee96917abf425cb2)
(cherry picked from commit a044cf60bd375f894b371fc87d148bf4704c05a8)