bcm2835_fb: add framebuffer device for Raspberry Pi

bcm2835_fb: add framebuffer device for Raspberry Pi

The framebuffer occupies the upper portion of memory (64MiB by
default), but it can only be controlled/configured via a system
mailbox or property channel (to be added by a subsequent patch).

Signed-off-by: Grégory ESTRADE <gregory.estrade@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Message-id: 1457467526-8840-4-git-send-email-Andrew.Baumann@microsoft.com
[AB: added Windows (BGR) support and cleanup/refactoring for upstream submission]
Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>