power: rk808: rename to rk8xx

Since this driver can be used for rk8xx series pmic,
let's rename rk808 to rk8xx, to make it clear.

Configs parts are done by sed -i "s/RK808/RK8XX/g" `grep RK808 -lr ./`

Signed-off-by: Jacob Chen <jacob-chen@iotwrt.com>