soc: imx: imx8m-blk-ctrl: Use genpd_xlate_onecell

Simplify driver by using genpd_xlate_onecell instead of
driver specific xlate function.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>