drivers: mailbox: qti-tcs: update verbosity of ftrace message

Make FTRACE messages from the mailbox driver more useful by printing the
mailbox name instead of the address where the data is being written to.
Get the mailbox names from the label property.

Change-Id: I509585653b68468c0d10dbdd183e395f05502b2a
Signed-off-by: Lina Iyer <ilina@codeaurora.org>