arm64: dts: apple: t8103: Add UART2

This UART is connected to the debug port of the WLAN module. It is
mostly useless, but makes for a good test case for runtime-pm without
having to unbind the console from the main system UART.

Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Hector Martin <marcan@marcan.st>