sunxi: video: Do not use CONFIG_SYS_MEM_TOP_HIDE for the framebuffer

Do not use CONFIG_SYS_MEM_TOP_HIDE for the framebuffer, instead override
board_get_usable_ram_top to make sure that u-boot is not relocated into the
area where we want to use the framebuffer, and patch the devicetree from
sunxi_simplefb_setup() to tell the kernel to not touch the framebuffer.

This makes u-boot properly see the framebuffer as dram, and initalize the
level 2 cache for it, fixing the very slow cfb scrolling problem.

As an added bonus this stops us from reserving the framebuffer when simplefb
is not used because an older kernel is booted, or hdp is used and no hdmi
cable was plugged in, freeing up the memory for kernel use in these cases.

Reported-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>