sunxi: Add sunxi_*_mem_reserve kernel cmdline options

This patch adds the following kernel commandline options:
sunxi_g2d_mem_reserve
sunxi_ve_mem_reserve
sunxi_fb_mem_reserve

These can be set to configure the amount of RAM (in MB) to reserve for the
named sunxi drivers. This is useful for kernels for ready-to-use disk images,
so that an user can choose to use various binary only drivers to use the
hardware blocks in question, while not making the memory unavailable for
general use for users who choose not to use these drivers, without requiring
a kernel recompile.

IE I'm currently using the following since I don't use any binary drivers:
sunxi_g2d_mem_reserve=0 sunxi_ve_mem_reserve=0 sunxi_fb_mem_reserve=16

Giving me an additional 112 MB of extra memory.

Note that:
1) This patch also allows making the reservations larger in an easy way :)
2) This patch removes the comments stating that memory won't be reserved if
the block in question is disabled in the fex file, since this was not true
before this patch, and still is not true after this patch.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>