apm821xx: disable MX60(W) due to kernel size

disables the MX60(W) from being built by the builders for now.
But there's an effort to bring it back:
<openwrt#4617>

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

target/linux/apm821xx/image/nand.mk

@@ -41,6 +41,7 @@ endef
 TARGET_DEVICES += meraki_mr24
 
 define Device/meraki_mx60
+  DEFAULT := n
   DEVICE_VENDOR := Cisco Meraki
   DEVICE_MODEL := MX60/MX60W
   DEVICE_PACKAGES := kmod-spi-gpio kmod-usb-ledtrig-usbport kmod-usb-dwc2 \