soc: imx: imx8m-blk-ctrl: set power device name

Set the name for the virtual power device to the name of the attached
blk-ctrl domain. Makes the debug output for the power domains a lot
more pleasant to read.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>