arm: zynq: Enable setup board name for different boards

There is no need to use zynq name as SYS_BOARD for all boards.
The patch is adding an option to change it.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

arch/arm/mach-zynq/Kconfig

@@ -36,6 +36,7 @@ config ZYNQ_DDRC_INIT
 	  want to skip ddr init and this option is useful for it.
 
 config SYS_BOARD
+	string "Board name"
 	default "zynq"
 
 config SYS_VENDOR