plat: intel: Fix mailbox send_cmd issue

There are a few issues in mailbox that needs to be fixed.
- Send doorbell after an indirect cmd
- Do not ring doorbell when polling mailbox response as it should've been
sent by send_cmd
- remove unneeded cmd_free_offset check
- Fix mailbox initialization
- Fix get_config_status returning a wrong status when the status is busy
- Add command length in mailbox command header

Signed-off-by: Tien Hock, Loh <tien.hock.loh@intel.com>
Change-Id: If613e2ca889a540a616c62d69ad0086a7cd46536