Add ffs.mtp.ready triggers to mtp configs.

This allows devices to use functionfs for mtp and ptp instead
of the ordinary functions. The property is set regardless of
which function is enabled so mtp and ptp functions will continue
to work.

Bug: 33220530
Test: use mtp with ffs and non ffs devices
Change-Id: I4527e153bd10ef1e262e487482ae455835d1c698