soc: ti: add firmware file name as part of the driver

Currently firmware file name is included in the DTS. This is not scalable
as user has to change the DTS if they need upgrade to a new firmware.
Instead, add the firmware file name in the driver itself. As long as there
is no API change, new firmware upgrade is easy and require no driver
change. User is expected to copy the firmware image to the file system
and add a sym link to the new firmware for doing an upgrade. Driver add
a array of firmware file names to search for the available firmware blobs.
This scheme also prepare the driver for future changes to API if ever
happens. In such case it is assumed that driver needs to change to
accommodate the new firmware and new firmware file name will get added to
the array.

Also update the DT document to remove the firmware attribute and add
description about firmware in the driver documentation.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>