x86: use invd instead of wbinvd in real mode start code

I do not know why the boot code immediately after the system reset
should write-back the cache content. I think the cache invalidation
should be enough.

I tested this commit with qemu-x86_defconfig, and it worked for me.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/cpu/start.S

@@ -50,7 +50,7 @@ _x86boot_start:
 	movl	%cr0, %eax
 	orl	$(X86_CR0_NW | X86_CR0_CD), %eax
 	movl	%eax, %cr0
-	wbinvd
+	invd
 
 	/*
 	 * Zero the BIST (Built-In Self Test) value since we don't have it.

arch/x86/cpu/start16.S

@@ -28,7 +28,7 @@ start16:
 	movl	%cr0, %eax
 	orl	$(X86_CR0_NW | X86_CR0_CD), %eax
 	movl	%eax, %cr0
-	wbinvd
+	invd
 
 	/* load the temporary Global Descriptor Table */
 data32 cs	lidt	idt_ptr