arm: dts: apple: Add preliminary device trees

Add preliminary device trees for the Apple M1 mini (2020) and
Apple M1 Macbook Pro 13" (2020).  Device tree bindings for
the Apple M1 SoC are still being formalized and these device
trees will be synchronized with the Linux kernel as needed.

The device trees in this commit are based on the initial Apple
M1 device trees from Linux 5.13, nodes for dart, pcie, pinctrl,
pmgr, usb based on bindings on track for inclusion in Linux
5.15 and 5.16 and nodes for i2c, mailbox, nvme, pmu, spmi and
watchdog that don't have a proposed binding yet.

These device trees are provided as a reference only as U-Boot
uses the device tree passed by the m1n1 bootloader.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Simon Glass <sjg@chromium.org>