Add support for static memory reserved from Device Tree. Since we're
using GEM buffers backed by CMA, it is interesting to have an option to
specify the CMA area where the GEM buffers will be allocated.

v2: Don't abort probe if reserved memory cannot be obtained. The driver
    will still work fine provided the kernel configuration is sane.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200926170501.1109197-3-paul@crapouillou.net