checkpatch: Ignore ENOSYS warnings

There are no system calls in U-Boot, but ENOSYS is still allowed (and
preferred since 42a2668743 ("dm: core: Document the common error codes")).
Silence this warning.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

.checkpatch.conf

@@ -26,6 +26,9 @@
 # addresses are __aligned(2)".
 --ignore PREFER_ETHER_ADDR_COPY
 
+# ENOSYS is a conventionally used error, even though U-Boot lacks system calls.
+--ignore ENOSYS
+
 # A bit shorter of a description is OK with us.
 --min-conf-desc-length=2