bcm2835_property: implement "get board revision" query

Return a valid value from the BCM2835 property mailbox query "get board
revision". This query is used by U-Boot. Implementing it fixes the first
obvious difference between qemu and real HW.

The value returned is currently hard-coded to match the RPi2 I own. Other
values are legal, e.g. different board manufacturer field values are
likely to exist in the wild.

Cc: Andrew Baumann <Andrew.Baumann@microsoft.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Message-id: 1454993910-24077-1-git-send-email-swarren@wwwdotorg.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
