soc: imx: imx8m-blk-ctrl: Fix IMX8MN_DISPBLK_PD_ISI hang

The imx8mn clock list for the ISI lists four clocks, but DOMAIN_MAX_CLKS
was set to 3.  Because of this, attempts to enable the fourth clock failed,
threw some splat, and ultimately hung.

Fixes: 7f511d51 ("soc: imx: imx8m-blk-ctrl: add i.MX8MN DISP blk-ctrl")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>