zynq: spl: Add vectors section to linker script

The vectors section contains the _start symbol which is used as the
program entry point. Add it to the linker script in same fashion as done
for regular u-boot. This allows for correct generation of an spl elf
with a non-zero entry point.

Similar change was applied to sunxi platform in
"sunxi: Fix u-boot-spl.lds to refer to .vectors"
(sha1: 9e5f80d823e3fd2a685b10ecf02009e34b86cff9).

This also allows for placement of the vector table at the hivecs
location by setting the TEXT_BASE to 0xffff0000.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>