xilinx: common: Change bootm_size variable setting

Linux kernel for arm32 requires dtb and initrd to be placed in low memory
to work properly. This requirement is described in chapter 4b) and 5) in
Linux documentation (Documentation/arm/booting.rst).

There is an issue on arm32 with 2GB of memory that bootm_size is bigger
than Linux lowmem (for example with VMSPLIT_3G). That's why limit bootm
size on these systems not to be above 768MB.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>