Allow Raspberry Pi platform and drivers to be configured with upstream DTBs.

• Added more compatibility strings to drivers not yet converted
• Added new RPI platform code compatibility string to match the ones used upstream
• Adapted RPI and RPI2 DTS to match the new platform code compatibility string

The goal is to use the upstream DTBs as a replacement for our custom one.
This is now possible with these changes.

Additionally, as the RPI firmware automatically chooses the right DTB for
us, this would allow to have one common armv6 kernel for RPI0 and RPI1
(BCM2835-based), and one common armv7 kernel for RPI2 v1.1 (BCM2836-based),
and RPI2 v1.2 / RPI3 (BCM2837-based).

Submitted by: Sylvain Garrigues <sylgar@gmail.com>
Differential Revision: https://reviews.freebsd.org/D12360