arm64: dts: imx: Add missing imx8mm-beacon-kit.dtb to build

The imx8mm-beacon-kit.dtb was never added to dtbs-y and wasn't getting
built. Fix it.

Fixes: 593816fa

 ("arm64: dts: imx: Add Beacon i.MX8m-Mini development kit")
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>