arm64: dts: imx8mm-kontron-n801x-som: do not allow to switch off buck2

The buck2 output of the PMIC is the VDD core voltage of the cpu.
Switching off this will poweroff the CPU. Add the 'regulator-always-on'
property to avoid this.

Fixes: 8668d8b2

 ("arm64: dts: Add the Kontron i.MX8M Mini SoMs and baseboards")
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>