arm64: dts: imx8mn-var-som: fix missing PMIC's interrupt line pull-up

The PMIC's interrupt is level low and should be pulled up.  The PMIC's
device node had pinctrl-0 property but it lacked pinctrl-names which
is required to apply the pin configuration.  The actual problem in DTS
was pointed out by Felix Radensky from Variscite.

Reported-by: Felix Radensky <felix.r@variscite.com>
Fixes: ade0176d ("arm64: dts: imx8mn-var-som: Add Variscite VAR-SOM-MX8MN System on Module")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Robin Gong <yibin.gong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>