mt8173: Add support for new watchdog SMC

mt8173: Add support for new watchdog SMC

This patch adds support for a new SMC that can be used to control the
watchdog. This allows for a cleaner separation of responsibilities where
all watchdog operations have to go through Trusted Firmware and we could
no longer have kernel and firmware poking concurrently at the same
register block.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Evan Benn <evanbenn@chromium.org>
Change-Id: I4844a3559d5c956a53a74a61dd5bc2956f0cce7b