arm64: zynqmp: allow overriding board name

There is no need to use zynqmp name as SYS_BOARD for all boards.
The patch is adding an option to change it.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Link: https://lore.kernel.org/r/20211020152518.3511912-1-liambeguin@gmail.com


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