ARM64: zynqmp: Add missing SPL dependency for boot.bin generation

boot.bin file is generated only when SPL is selected.
Reflect this depency in Kconfig.

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

arch/arm/cpu/armv8/zynqmp/Kconfig

@@ -43,6 +43,7 @@ config SYS_CONFIG_NAME
 
 config BOOT_INIT_FILE
 	string "boot.bin init register filename"
+	depends on SPL
 	default ""
 	help
 	  Add register writes to boot.bin format (max 256 pairs).