watchdog: Add new arm_smc_wdt watchdog driver

This patch adds a watchdog driver that can be used on ARM systems
with the appropriate watchdog implemented in Secure Monitor firmware.
The driver communicates with firmware via a Secure Monitor Call.
This may be useful for platforms using TrustZone that want
the Secure Monitor firmware to have the final control over the watchdog.

This is implemented on mt8173 chromebook devices oak, elm and hana in
arm trusted firmware file plat/mediatek/mt8173/drivers/wdt/wdt.c.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Evan Benn <evanbenn@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: Xingyu Chen<xingyu.chen@amlogic.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200505131242.v6.2.Ia92bb4d4ce84bcefeba1d00aaa1c1e919b6164ef@changeid
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>