ARM: bcm2835: dt: Add vendor prefix for Raspberry Pi

Since the prefix is already in use, we need to add it in the
vendor list.

Reviewed-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>