delay_timer: correct sparse issues and warnings ...

Use NULL instead of 0 where required.
Include headers to have the prototype of the functions.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>

Use SPDX license identifiers ...

To make software license auditing simpler, use SPDX[0] license
identifiers instead of duplicating the license text in every file.

NOTE: Files that have been imported by FreeBSD have not been modified.

[0]: https://spdx.org/

Change-Id: I80a00e1f641b8cc075ca5a95b10607ed9ed8761a
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>

Implement generic delay timer ...

Add delay timer implementation based on the system generic counter.
This either uses the platform's implementation of
`plat_get_syscnt_freq()` or explicit clock multiplier/divider values
provided by the platform.

The current implementation of udelay has been modified to avoid
unnecessary calculations while waiting on the loop and to make it
easier to check for overflows.

Change-Id: I9062e1d506dc2f68367fd9289250b93444721732