The OMAP mailbox driver and its existing clients (remoteproc
for OMAP4+) are adapted to use the generic mailbox framework.

The main changes for the adaptation are:
  - The tasklet used for Tx is replaced with the state machine from
    the generic mailbox framework. The workqueue used for processing
    the received messages stays intact for minimizing the effects on
    the OMAP mailbox clients.
  - The existing exported client API, omap_mbox_get, omap_mbox_put and
    omap_mbox_send_msg are deleted, as the framework provides equivalent
    functionality. A OMAP-specific omap_mbox_request_channel is added
    though to support non-DT way of requesting mailboxes.
  - The OMAP mailbox driver is integrated with the mailbox framework
    through the proper implementations of mbox_chan_ops, except for
    .last_tx_done and .peek_data. The OMAP mailbox driver does not need
    these ops, as it is completely interrupt driven.
  - The OMAP mailbox driver uses a custom of_xlate controller ops that
    allows phandles for the pargs specifier instead of indexing to avoid
    any channel registration order dependencies.
  - The new framework does not support multiple clients operating on a
    single channel, so the reference counting logic is simplified.
  - The remoteproc driver (current client) is adapted to use the new API.
    The notifier callbacks used within this client is replaced with the
    regular callbacks from the newer framework.
  - The exported OMAP mailbox API are limited to omap_mbox_save_ctx,
    omap_mbox_restore_ctx, omap_mbox_enable_irq & omap_mbox_disable_irq,
    with the signature modified to take in the new mbox_chan handle instead
    of the OMAP specific omap_mbox handle. The first 2 will be removed when
    the OMAP mailbox driver is adapted to runtime_pm. The other exported
    API omap_mbox_request_channel will be removed once existing legacy
    users are converted to DT.

Signed-off-by: Suman Anna <s-anna@ti.com>
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>