sunxi: sunxi-common.h: Reduce bootm_size to take the framebuffer into account

sunxi: sunxi-common.h: Reduce bootm_size to take the framebuffer into account

Before video output support can be enabled on the A13-OLinuXinoM, bootm_size
must first be reduced to take into account that the framebuffer is shaved of
the top of the DRAM. For other boards this is not an issue since bootm was set
to 256M and all boards have at least 512M except for the A13-OLinuXinoM.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>