Correct SPL use of SYS_MEM_RSVD_FOR_MMU

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_SYS_MEM_RSVD_FOR_MMU defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/xilinx/versal-net/board.c b/board/xilinx/versal-net/board.c
index 7ff2c62..6724c72 100644
--- a/board/xilinx/versal-net/board.c
+++ b/board/xilinx/versal-net/board.c
@@ -195,7 +195,7 @@
 {
 	int ret;
 
-	if (CONFIG_IS_ENABLED(SYS_MEM_RSVD_FOR_MMU))
+	if (IS_ENABLED(CONFIG_SYS_MEM_RSVD_FOR_MMU))
 		ret = fdtdec_setup_mem_size_base();
 	else
 		ret = fdtdec_setup_mem_size_base_lowest();