watchdog: aspeed: Allow configuring for alternate boot

Allow the device tree to specify a watchdog to fallover to
the alternate boot source.

The aspeeed watchdog can set a latch directing flash chip select 0 to
chip select 1, allowing boot from an alternate media if the watchdog
is not reset in time.  On the ast2400 bank 1 also goes to flash bank 1,
while on the ast2500 the chip selects are swapped.

Also clear the secondary boot bit during the machine restart operation.
Otherwise, the system will switch to the alternate boot after every
reboot, which is not desired.

Signed-off-by: Milton Miller <miltonm@us.ibm.com>
Signed-off-by: Eddie James <eajames@linux.vnet.ibm.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>