Davinci: watchdog reset separation across socs

The earlier watchdog reset mechanism had a couple of limitations.  First, it
embedded a reference to "davinci_wdt_device" inside common code.  This
forced all derived platforms (da8xx and tnetv107x) to define such a device.
This also would have caused problems in including multiple socs in a single
build due to symbol redefinition.

With this patch, davinci_watchdog_reset() now takes the platform device as an
argument.  The davinci_soc_info struct has been extended to include a reset
function and a watchdog platform_device.  arch_reset() then uses these
elements to reset the system in a SoC specific fashion.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Tested-by: Sandeep Paulraj <s-paulraj@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>