armv7a: irq_handler() and fiq_handler() prototypes

Add function prototypes for the irq_handler() and fiq_handler()
functions.  These must be implemented for each chip, so place them in
arch/chip.h, and mention them in the porting docs.

Change-Id: I2ac781a6a4dc451db5dc64adc53dfa1ac95148bd
sched: provide sched_system_tick()

Provide sched_system_tick(), which performs the standard system tick
functions.  It is called at CONFIG_SYSTICK_FREQ by an arch-provided
timer.

This eliminates each arch being responsible for performing individual
system tick functions at each tick.  Rather, it provides a single
function that is called at each tick.

Change-Id: I8b514f9831aa02081564d2732dac23fbad8813b8
sched: use arch_sched_start_system_tick() to begin system ticks

Only ARMv7-M uses the actual "SysTick" timer for performing system
ticks.  ARMv7-A devices must use a general purpose timer.  Rather than
having those devices use the init_systick() symbol, intended for the
official SysTick timer, use arch_sched_start_system_tick() to enable
whatever timer is necessary for system ticks.

On ARMv7-M, this simply initializes the SysTick timer.

On ARMv7-A, the buck is passed one level further, to the chip, which
must provide a timer for system ticks.

Change-Id: Ib2c26c57edd65c69ca7fde62440241c7b97a0fe8