soc: imx: gpcv2: handle additional power-down bits in handshake register

Some of the i.MX8MQ domains have an additional control bit in the PU
handshake (HSK) register. Documentation about this bit is a bit sparse
at the moment, but it seems that it controls a power-down request to
the AMBA domain bridge (ADB-400) attached to those domains.

As the documentation doesn't desribe the usage of this bit yet, handle
it in the same way as done in the ATF implementation.

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