watchdog: Add Realtek Otto watchdog timer

Realtek MIPS SoCs (platform name Otto) have a watchdog timer with
pretimeout notifitication support. The WDT can (partially) hard reset,
or soft reset the SoC.

This driver implements all features as described in the devicetree
binding, except the phase2 interrupt, and also functions as a restart
handler. The cpu reset mode is considered to be a "warm" restart, since
this mode does not reset all peripherals. Being an embedded system
though, the "cpu" and "software" modes will still cause the bootloader
to run on restart.

It is not known how a forced system reset can be disabled on the
supported platforms. This means that the phase2 interrupt will only fire
at the same time as reset, so implementing phase2 is of little use.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/6d060bccbdcc709cfa79203485db85aad3c3beb5.1637252610.git.sander@svanheule.net


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>