watchdog: aspeed: Fix translation of reset mode to ctrl register

Assert RESET_SYSTEM bit for any reset and set MODE field from reset
type.

The watchdog control register has a RESET_SYSTEM bit that is really
closer to activate a reset, and RESET_SYSTEM_MODE field that chooses
how much to reset.

Before this patch, a node without these optional property would do a
SOC reset, but a node with properties requesting a cpu or SOC reset
would do nothing and a node requesting a system reset would do a
SOC reset.

Fixes: b7f0b8ad ("drivers/watchdog: ASPEED reference dev tree properties for config")
Signed-off-by: Milton Miller <miltonm@us.ibm.com>
Signed-off-by: Eddie James <eajames@linux.vnet.ibm.com>
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>