rockchip: mkimage: fix wrong range of rc4 encoding for boot image

The rc4 encoding should cover spl header as well, and the file_size
contains spl header too.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/tools/rkimage.c b/tools/rkimage.c
index ae50de5..1c5540b 100644
--- a/tools/rkimage.c
+++ b/tools/rkimage.c
@@ -18,7 +18,7 @@
 	memcpy(buf, rkcommon_get_spl_hdr(params), RK_SPL_HDR_SIZE);
 
 	if (rkcommon_need_rc4_spl(params))
-		rkcommon_rc4_encode_spl(buf, 4, params->file_size);
+		rkcommon_rc4_encode_spl(buf, 0, params->file_size);
 }
 
 static int rkimage_check_image_type(uint8_t type)