arm64: zynqmp: Do not define do_reset() if sysreset is enabled

The SPL can also be compiled with sysreset drivers just fine, so
update the condition to cater for that option.
The same change was done by commit efa1a62a ("ARM: imx8m: Do not define
do_reset() if sysreset is enabled").

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