spl: x86: Correct the binman symbols for SPL

These symbols are incorrect, meaning that binman cannot find the
associated entry. This leads to errors like:

binman: Section '/binman/simple-bin': Symbol '_binman_spl_prop_size'
   in entry '/binman/simple-bin/u-boot-spl/u-boot-spl-nodtb':
   Entry 'spl' not found in list (mkimage,u-boot-spl-nodtb,
   u-boot-spl-bss-pad,u-boot-spl-dtb,u-boot-spl,u-boot-img,main-section)

Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/x86/dts/u-boot.dtsi

@@ -37,7 +37,7 @@
 	u-boot-tpl-dtb {
 	};
 #endif
-	spl {
+	u-boot-spl {
 		type = "u-boot-spl";
 		offset = <CONFIG_X86_OFFSET_SPL>;
 	};