timeout_init_us(some_timeout_us); returns a reference to detect
timeout for the provided microsecond delay value from current time.

timeout_elapsed(reference) return true/false whether the reference
timeout is elapsed.

This change is inspired by the OP-TEE OS timeout resources [1].

 [1] https://github.com/OP-TEE/optee_os/blob/3.4.0/core/arch/arm/include/kernel/delay.h#L45

Change-Id: Id81ff48aa49693f555dc621064878417101d5587
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>