Add a delay timer driver for the ARM SP804 dual timer.

This driver only uses the first timer, called timer 1 in the
SP804 Technical Reference Manual (ARM DDI 0271D).

To use this driver, the BSP must provide three constants:

*   The base address of the SP804 dual timer
*   The clock multiplier
*   The clock divider

The BSP is responsible for calling sp804_timer_init(). The SP804
driver instantiates a constant timer_ops_t and calls the generic
timer_init().

Change-Id: I49ba0a52bdf6072f403d1d0a20e305151d4bc086
Co-authored-by: Dan Handley <dan.handley@arm.com>