drivers: mailbox: rpmh-mailbox: rename file to match convention

Rename the qti-tcs.c file to match convention used in the
drivers/mailbox folder. Also rpmh-mailbox is a better name than
tcs-mailbox.

Change-Id: I3a398f2b41a3ef579b5661f2fe31a635a390237a
Signed-off-by: Lina Iyer <ilina@codeaurora.org>