units: Add Watt units

[ Upstream commit 2ee5f8f05949735fa2f4c463a5e13fcb3660c719 ]

As there are the temperature units, let's add the Watt macros definition.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Stable-dep-of: 3ef79cd14122 ("serial: sc16is7xx: set safe default SPI clock frequency")
Signed-off-by: Sasha Levin <sashal@kernel.org>