thermal: Compatible strings for bcm2836, bcm2837

thermal: Compatible strings for bcm2836, bcm2837

The upstream dt-bindings documentation for bcm2835-thermal (which
exists even though the driver isn't upstreamed) says to use
dedicated compatible strings on bcm2836 and bcm2837, even though
the downstream driver doesn't support them. The Pi2 DTB uses
"brcm,bcm2836-thermal", so the driver doesn't load. The Pi3 DTB
doesn't override the base value, but the arm64 Pi3 support uses "brcm,bcm2837-thermal".

Solve the documentation problem by adding "brcm,bcm2836-thermal" and
"brcm,bcm2837-thermal" as alternative compatible strings for the
bcm2835-thermal driver.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>