Add bcm2710-rpi-cm3.dtb to the list of DTBs being added.

This allows to boot out of the box on the RPI COmpute Module 3 with 32G
of eMMC.

Tested by: imp confirmed .dtb is in the rpi-firmware pkg
Reviewed by: gjb@, imp@
Pull Request: https://github.com/freebsd/freebsd-src/pull/452

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