arm64: dts: imx93: Add the bbnsm dts node

Add the bbnsm node for RTC & ON/OFF button support

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>