x86: Correct XIP_ROM_SIZE

This should default to the size of the ROM for faster execution before
relocation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index ebf72b3..7d007bb 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -53,7 +53,7 @@
 
 config XIP_ROM_SIZE
 	hex
-	default 0x10000
+	default ROM_SIZE
 
 config CPU_ADDR_BITS
 	int