x86: Disable CAR before relocation on platforms that need it

For platforms with CAR we should disable it before relocation. Check if
this function is available and call it if so.

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

arch/x86/cpu/start.S

@@ -205,6 +205,14 @@ board_init_f_r_trampoline:
 	/* Setup global descriptor table so gd->xyz works */
 	call	setup_gdt
 
+	/* Set if we need to disable CAR */
+.weak	car_uninit
+	movl	$car_uninit, %eax
+	cmpl	$0, %eax
+	jz	1f
+
+	call	car_uninit
+1:
 	/* Re-enter U-Boot by calling board_init_f_r */
 	call	board_init_f_r