arm64: dts: imx8mp-verdin: add 88W8997 serdev to uart4

Use the serdev feature to load the driver for the 88W8997 bluetooth
driver.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>