watchdog: octeontx_wdt: Add support for start and stop

This patch enhances the Octeon TX/TX2 watchdog driver to fully enable
the WDT. With this changes, the "wdt" command is now also supported
on these platforms.

Signed-off-by: Suneel Garapati <sgarapati@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Aaron Williams <awilliams@marvell.com>
Cc: Suneel Garapati <sgarapati@marvell.com>
Cc: Chandrakala Chavva <cchavva@marvell.com>