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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

The API is simple. The BSP or specific timer driver creates an
instance of timer_ops_t, fills in the timer specific data, then calls
timer_init(). The timer specific data includes a function pointer
to return the timer value and a clock multiplier/divider. The ratio
of the multiplier and the divider is the clock frequency in MHz.

After that, mdelay() or udelay() can be called to delay execution for
the specified time (milliseconds or microseconds, respectively).

Change-Id: Icf8a295e1d25874f789bf28b7412156329dc975c
Co-authored-by: Dan Handley <dan.handley@arm.com>