watchdog: hpwdt: Modify to use watchdog core.

Follow Documentation/watchdog/convert_drivers_to_kernel_api.txt to
convert hpwdt from legacy watchdog driver to use the watchdog core.

Removed functions: hpwdt_open, hpwdt_release, hpwdt_write, hpwdt_ioctl
Removed data structures: hpwdt_fops, hpwdt_miscdev, watchdog_device
Modified functions: hpwdt_start, hpwdt_stop, hpwdt_ping, hpwdt_gettimeleft
Added functions: hpwdt_settimeout
Added structures: watchdog_device

Update Kconfig file to show that hpwdt now selects WATCHDOG_CORE.

Signed-off-by: Jerry Hoemann <jerry.hoemann@hpe.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>