Add FreeBSD layer for AM335x-based devices DTS files

[gonzo]

Summary

After switching to Linux DTS files for AM335x-base boards,
some of the devices are no longer available on FreeBSD, namely:

• PMIC, power controller: irq property is missing in upstream DTS
• PRUSS: node is missing in upstream DTS
• SPI and I2C2: not enabled in upstream DTS

Users of earlier builds would expect all these devices to be present
in later releases as well, so we can't just stop supporting them.

Possible solution:

• Add $SRCBASE/sys to include path in make_dtb.sh so FreeBSD-specific DTS files in boot/fdt/dts can include files in gnu/dts using #include <gnu/dts/..>
• Bring back FreeBSD-specific beaglebone-black.dts and beaglebone.dts that apply freebsd-specific layer to the upstream am335x-bone*.dts files

[gonzo]

Do not use the same name as upstream DTS files and add UPDATING notice
about compatibility breakage

[manu]

Possible solution (cont):

• Upstream the changes that make sense.

I2C2 is enabled upstream, this is where the cape eeprom is (see am335x-bone-common.dts)
IIRC spi was enabled by loos@ but I don't know why.