davinci: Fix watchdog reset code

The davinci reset routine, davinci_watchdog_reset(), sets the
TCR register instead of the TGCR register as it should to put
the WDT into its "Initial State".

It also writes the WDTCR register without the proper WDKEY
which is pointless since the register will be write-protected.

Signed-off-by: David Griego <dgriego@mvista.com>
Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>