arm: dts: ti: Fix mmc3 instance by setting it to disabled

DTS Import of Linux 5.3 added a patch that rework the L3 mmc instance
in the AM335x SoC but removed the status = 'disabled' on the node.
This cause the kernel to probe the device even if the board doesn't
have this mmc used and since we don't correctly activate the clock
for this module we panic with an external data abort.
Beaglebone(s) don't have this device anyway so simply disabling it.
Patch for the DTS was sent upstream.
https://patchwork.kernel.org/patch/11176921/

PR: 241089
Reported by: phk